For cross country & showjumping, I use either medium sized dome studs if soft going, or medium pointys if it's firm (2 in each hind) none in the front.
I generally only use them behind (2 in each). Depends on the ground though, sometimes don't need them. I'll only use in front if it's really bad but even then only small studs (and 2 in each). Good all round studs are the small conical ones from supastuds. They can cope with slipping in front a lot more than behind.