Which interest rates are you talking about?
If it's student loans, then most likely they factor into the amount the school thinks you need for COA. Most of the time, the increase in interest rate of federal loans do not impact how much you receive. That being said, it's your responsibility on how much you're willing to take at that percentage. The less loans you take out, the easier it is for you to pay off and the less of compounded interest you have to pay on your principle after school. If you're talking about trying to refinance, then that depends on how much you're earning at the time. If you take out a certain amount of loans a year and realize you have a surplus of money, GIVE IT BACK THAT YEAR. Do not keep it.
Rate of returns has low volatility, but your interest rates in loans do not impact your Roth/401K/etc. That's how much you can afford and after doing the math, you will know. For instance, if your student loan after med school is $250k at a 6.8% interest rate. And you enter a stock or IRA for avg 8% gain, then it's possible that you can invest a portion of your earnings into an IRA/stock. If you work somewhere that matches your 401K, then I'd suggest doing that cause that is guaranteed free money that you would be giving up.