Depends. Obviously you won't have any published research by the time it comes to applying for residency programs, so that won't be beneficial. Then again, working at the NIH has some sort of prestige attached.
If you are hoping it will help you land a more prestigious residency, if you get some good letters from well-known writers there, it might; but not the experience in and of itself. Remember that research is not required or even important in all residency programs.
You may consider taking a year off DURING residency to work at the NIH (or elsewhere). By that time you might have a better idea of your interests, can use the break to relax a bit and boost your CV. If you have a funded position a residency PD may very well grant you the lab time.