They are not going to do a credit check.
If they did, they would have already had to do it before ranking you for the Match. You Matched. That means that you have the spot. You are bound to show up for it, and they are bound to let you come. Your credit is that you completed medical school. Your financial credit score is irrelevant for your job as a first year intern.
Checking credit is a way to ascertain the character of an unknown individual who wants to do business with you... how likely are they to fail in their obligations? You are not an unknown. You have been more thoroughly vetted several times over than just about anyone in society in order to get to where you are now. You've had more background checks, more requirements to demonstrate follow through on commitments, etc.
You will want to clean your credit record up. As others have said, it might cause you some problems with securing housing (probably not... people with bad credit get apartments all the time, though they may have a issues with particular landlords. Someone will be happy to take your money.) And you may want a job later on where they will run a score on you.
You will also want to clean up your credit because you are entering a profession where people generally invest a lot of trust in you. Just as a point of personal pride, you will want all aspects of your life to demonstrate your trustworthiness. If you still owe debts that you haven't been able to pay, make a point of paying them off. It will take a few years for a poor payment history to fall off the report, but it will with time.
I have some pretty crazy black marks on my credit history. I had a business and ran into a dispute with a commercial landlord such that I chose to ruthlessly default on debt for a period of time in order to force his hand and prevent him from taking adverse action against me. He was being absurd and doing things to the property that were making it impossible for me to conduct business, and I was looking at a lengthy court battle... just as I was getting ready to liquidate the business and start medical school. So, I ceased paying any creditors except my mortgage, creating a situation where I could have filed bankruptcy. As my largest creditor, who had been receiving an enormous chunk of my cashflow, he would have been subject to a 90 day clawback where the bankruptcy court would have required him to return 3 months of rent to be held in escrow and possibly redistributed pro-rata to my other creditors. I was putting all the money I wasn't paying on bills into an account that would have also been used to pay everyone, so everyone was going to get their money in the end, including the landlord... but I could have caused a large and troublesome disruption to his cash flow in the process. I can't explain it any better than that I was basically playing a game of financial chicken with him, to see who flinched first. It was him. He stopped resisting my efforts to sell my business and end my dealings with him, and everyone got paid without me having to actually finish the bankruptcy filing. But my personal credit rating shows that I just stopped paying bills for several months. My score is still in the low 600s after several years of consistent on-time payments.
Boy howdee did that make getting PLUS loans to pay for school challenging. Not impossible, though. Just problematic. I needed a cosigner for a couple of years.
I was able to pass a credit check for an apartment. And to get issued several new lines of credit which are going to make moving to another state a little easier to accomplish. If I can have a few tens of thousands of dollars of outrageous adverse commercial credit report entries, I'm sure you are going to be fine with your consumer level charge offs or whatever.
