I find thumb rests usually get in my way, just like the film advance lever on my film M and other cameras often do. With those, all except the Leicaflex SL, I end up folding the lever in most of the time unless having it at the stand off position is essential to keeping the meter turned on or unlocking the shutter release (like it has been on a couple of them).
The Leicaflex SL is the only camera I've ever had where the film wind lever has just the right stand-off and action so that I can leave it deployed when shooting and use it as a fast film advance ought to be used. I never use it as a thumb rest even with that camera ... My thumb rests on the body where it fits naturally with a well designed camera.
I have largish hands. The Leica M-D fits my hands beautifully, as did the SL and aforementioned Leicaflex SL. My recent re-acquisition, the Fuji GS645S Wide 60, is also a very good fit to my hands.
With smaller cameras, I often buy a half case to bulk them up a little bit and make them less tiring to hold steadily.