according to the pictures, it seems pretty rare.
You can't tell by how many pictures you find on the internet how rare or common it was. It's obvious that you would normally keep all your weapons pointed at the perceived threat. Therefore the cupola would be normally aligned with the main gun and thus you would get more photos of it. I've written my local American Legion Posts to try to make contact with former M-60 crewmen or maintenance folks just to satisfy my curiosity.
The point is the game allows you to open the hatch with the O key, no matter where the cupola is. You get this really stupid animation where the hatch opens and you seem to be ready to stick your head out but you never make it if the cupola is not lined up. It just confuses people that don't know this is a "feature". Either make it so you can stick your head out, no matter the alignment (as you could in real life) or get rid of the half baked animation.
The origin of this debate was that Andrey said that you couldn't turn out from the cupola hatch unless the cupola was aligned with the turret or unless you hung from the hatch like an ape. 3 pictures and a video prove you could. Posting a billion pictures of the cupola in alignment with commanders turned out doesn't change that.
The standing step, mentioned in the news video even shows that the tank was designed and built to do that. You didn't need to depend on that little folding seat that was fixed to the turret.