Like DA said, it really depends on your contract.
I know in my area, picking out a pasture is basically unheard of. But, generally, around here, pastures are large enough that it's really not an issue.
Of course, some areas of the pasture will end up looking "nasty" (my mare's poop "spot" is right by the gate...so helpful! Haha) but the majority of the pasture is pile-free and "clean".
Fabio - 13 year old Arabian/Lipizzaner gelding
Hazel - 14 year old Angora goat
Atticus - 4 year old LaMancha/Alpine cross goat
Rest peacefully, Lacey.