In PAYE interest will capitalize when you are not making PAYE-sized payments. When does this occur? When you are a student. If you defer / forebear during residency. If you are making really really big bucks as an attending and switch to the 10 year plan because it is cheaper.
In PAYE interest will not capitalize when you do make PAYE-sized payments. When does this occur? If you choose to make microscopic payments in residency. If you are a normal attending with a big loan and you can stay on the PAYE payment plan as an attending.
Why don't you go on over to the AAMC website and then in their loan calculator you can enter your loans, your projected resident salary, and your projected attending salary. You can play with repayment scenarios in there until you are blue in the face.
I would just add that you can make additional payments above and beyond the IBR/PAYE payment, while remaining in the program and retaining the repayment benefits.
My wife and I have quite a bit of debt between the two of us. My IBR payments in residency don't come close to paying off my interest accrued each year (which as mentioned, will be in limbo). I am, however, paying $10,000-$30,000 per year on my highest-interest loans to try and start shaving down my loans (though they will likely still grow because of interest accruing). I am not going into a high-paying specialty, and thus I will still be eligible for IBR when I am making an attending salary. At that point I'll still make the monthly IBR payment. But I will apply a LOT of my additional income towards quickly paying down my highest interest rate loans. In order, my targets are:
1) Private loans (variable rate, although much lower than federal loans right now
2) My wife's loans (she's not eligible for PSLF--might as well get rid of her loans first)
3) My higher interest federal loans.
If I'm lucky, the job I want will be at a non-profit/federal institution, and PSLF will still be around. That would mean that I will pay off my private loans and my wife's loans in their entirety (assuming she stops working to raise our children, which is something we're still debating). I will have likely paid off some of my federal loans. But then I will hopefully be able to benefit from PSLF if it does stick around. If it doesn't, I'm still planning on making more-than-the-minimum monthly payment, even once we get down to just my loans, because letting my loans snowball is about the scariest thing I can imagine.
Note that since I'll be in IBR essentially for the rest of my life (until I pay enough of our loans down to not qualify anymore) then none of my interest will ever capitalize (other than what already capitalized at the end of my grace period--there's no way to get around that unfortunately...). Once I have an attending salary and am making payments large enough, the government will first apply my payments towards just the interest (even though the interest is in limbo, interest is what always gets paid off first). Then eventually I'll start whittling at the principle (plus the interest that accrues each month)
I wouldn't get too caught up in the 10% thing with PAYE and loan capitalization--if you are going straight into PAYE and will be in it for the rest of your repayment it has absolutely no benefit for you. If you do that, the only time your loans will capitalize is at the start of repayment/end of your grace period. Unless you were in school an awfully long time or had very high interest rates, that 10% rule won't benefit you with that first interest capitalization event. But then your loans would never capitalize again--so I don't think it's something to concern yourself with.
Like sazerac has been saying--you are responsible for paying off your principle and any accrued interest. Some of that interest may be added to the principle (capitalization), but you will have to pay off everything. If it all gets forgiven, you will be taxed on the entire amount you owe Uncle Sam.
Once again--please consider making payments on top of the IBR/PAYE payments, if they don't cover your interest. You can selectively apply additional payments to your highest interest loans. If you are a resident and only have 50k in income, then true you won't be able to cover much more (but most residents can still handle paying extra unless maybe they're in NYC). If you as a PA are making ~100,000, then you will probably take home about 70k after taxes, maybe a little less or more depending on how good you are at taxes. I'd recommend living on 30k (that's quite a bit of post-tax dollars) and putting 40k towards your loans for at least 2-3 years. Live minimally. After three years you'll have paid off quite a bit of your loans. Then maybe start tapering down at that point--remember, paying more up-front is better than paying more later because of interest. True, under PAYE interest doesn't capitalize, but if you front load repayment then you can pay off whatever interest has accumulated and start paying down the principle, and paying down that principle has a 6-8% return on your investment, which is quite good. I think with a $100,000 salary, you should be able to pay off $200,000 without too much undue hardship, especially if you have/will have a spouse (just my personal opinion).
My personal plan is to do exactly that. If I make 200k, uncle sam and my state will take at least 70k, leaving me with ~120k. I'll then apply 60-80k towards my loans, and basically just live at the same level I am at right now for a few additional years. That'll give me peace of mind and the ability to start saving for my children's college, my retirement, etc. If in 2017, when I finish residency, it looks like PSLF is going to stick around and I'm still eligible, then I'll make just the minimum payments on my federal loans (and put that extra 60-80k towards my private and spouse's loans). But like I said before, it's a big gamble to be counting on forgiveness (and I don't want to pay those ridiculous taxes), so if PSLF disappears or if I get a job that doesn't qualify, I'm going to pay off my loans as fast as I can. I borrowed the money expecting to pay it all back, and sure, if I know I can benefit from forgiveness without a doubt I'll take advantage of it, but otherwise I'm paying back every dime as quick as possible. It's much easier to be poor when you're single/married without kids (or just starting to raise the family).