It depends on the program. For example, at the Cleveland Clinic, all of the resdents are REQUIRED to do a 6th year in research. At CWRU all six residents are admitted under the same NRMP number and they kind of hash it out among themselves to pick the two who do the lab year. Others like the University of Pittsburg have separate NRMP listings, or offer a research position outside of the match (Louisville does this).
Certainly these positions are more desirable to some and less desirable to others; I've been given the impression that volunteering to do a research year is not an easy way in to orthopaedics.
There's no one good blanket answer; contact the individual program.
Good luck.