Almost every time (except for a few exceptions) we have always had to wait at Ohana's even with PS Ressies.
I think once we got in without a wait. Usually we are waiting in the lobby with a million other people!
You could always make a PS, and then cancel them if you decide not to go. It depends on how bad you definately want to go. If you are not set either way then winging it might be the way to go. If you really HAVE to go, then the PS is definately the way to go.