I think scheduling is overblown in most instances. At least it used to be. Though, now with a lot of the programs letting you out earlier and earlier (It used to be only Scholl that was starting externships Jan/Feb of their 3rd year and nobody else was starting earlier than May or June), there may be something to not scheduling your "top choice" in the first month (assuming that month is March/April for example). I will say that the Scholl students real early in the year (Jan-March) when I was a resident were rarely considered as they were uniformly bad. However, I spent my first month (June) at a program I knew would give me hands on experience which I felt like I needed, in preparation for the remaining rotations. I did not think that I would end up wanting to go there for geography reasons so I didn't mind "wasting" that month if I was a less than stellar extern. It ended up being my first choice and I ended up being ranked #1 there. I was ranked in the top 2 at programs I rotated at in June and October, ranked 3-5 at my July and August programs (both took 2 residents), and well outside of that for the programs I clerked at in September and November.
I think making relationships with upperclassman, which may be easier said than done, is the most important aspect in terms of picking programs to clerk. I had the benefit of knowing a lot of older students from all of the podiatry schools thanks to APMSA, but it's not impossible without that. I did have a geographic area that I was focused on, and started there. It was large, but there aren't a ton of programs West of the Rockies so that eliminated a lot of programs that I didn't have to research or worry about. I don't think there is anything wrong with narrowing down your list based on geography right away since you can get good training in virtually every region of the country now. Once I had geography established, I basically followed upperclassmen who I trusted/respected. It may have helped that most of the programs I clerked at were "big name" or well known, but my thought process was, "if this program is good enough for Johnny or Suzy, then there has to be redeeming qualities to it. Whether or not I feel the same way about the program, I'm confident it offers good training." Again, I had personal relationships with these people, and that really ended up being most helpful since I had people at the program pulling for me and helping me, so to speak. Even if you don't really know the individuals who ended up matching at the programs you are externing at, getting information from individuals you trust/respect/etc. about programs they rotated at or ended up at is 100x more valuable than reading through those CASPR pages about a bunch of programs you know nothing about.
In short, find geographical area(s) you would prefer, if in fact there is a preference. This will decrease the amount of research you need to do right off the bat. Then, find upperclassmen you know and trust and pick their brains. If you have friendships and can extern at places they liked and/or matched at, even better.