Usually is it whatever the pro grew up with and got used to. Prince is more rounded - Head more rectangular - Dunlop and Wilson pretty similar in between etc. So most pros use a grip shape that is one of those -- although sometimes it will be a Dunlop or Wilson style on a Head stick etc. Usually when a P1 or company is making a "custom" handle it is usually based off one of the players own sticks that they like the grip shape on and they make them all to match that one.
And then sometimes the players will have a custom size or really get involved and make a total custom shape that suits said player. But the majority are pretty much using just what they grew up using and are most comfortable with.
I'll get you a toe by this afternoon--with nail polish.