Now that it's getting close to Thanksgiving, we're running a contest to hear advice you've received that you're most thankful for! This can be any type of advice and the advice with the most reactions will win!
During first year, only do it if you are doing well in your courses. Some people find it manageable, but others don't. Definitely line up something for the summer after first year, if you are so inclined -- if it is going well, you can consider continuing the project into your second year. And you can always take a research year off if you feel you need research for your residency goals, but did not have time during the academic year.
I was paid to do bench research between the M1 and M2 years. I have continued my participation in the same lab on a volunteer basis, which gives me a lot of control over the hours I make myself available. It remains to be seen whether I'm shooting myself in the foot by spending possible study time there.