It's not strictly necessary to take USD with you before going to Cambodia, since the ATMs dispense them.

Of course, depending on the transaction fees and what currency you have already, it may be cheaper to just bring some with you.

Just remember:
1 Cambodia is not a place to be leaving cash in the hotel safe.
2 Larger notes need to be in pristine condition.
3 Every time you present a $20 or $50 note, even in a large supermarket, they will ask if you have something smaller. Be prepared to say no, so that you do have some smaller notes for when you need them.