I've run my GP100 (with a Millett adjustable rear sight) and Security Six (with an adjustable Ruger rear sight) without any incident in both a Bianchi UM84R flap holster and the Security Six in a Hunter leather holster where the rear sight is enclosed by the leather fairly tightly; while in the back of my mind I've occasionally wondered if they's adversely impact against the rear sights/zero, in reality, I've had no adverse results.
My thought is that as long as the sight adjustment screws and springs are in good shape, it shouldn't be an issue. I don't particularly worry about it anymore.
In all honesty, though, I've only very occasionally used either of the holsters, and never for a period exceeding 6-8 hours.
Best, Jon