Yep, that's only caveat. If you're on T-Mo prepaid, you have to be "all-in" with Google Voice to use their VM reliably because they don't give you call forwarding with prepaid. But of course, this isn't a problem, provided that you're only giving out your GV number.
The only thing that turning off carrier voicemail actually does is prevent carrier VM from ever picking up when someone calls the actual handset number. This is mostly a measure to prevent the annoying VM notification that only goes away when you actually check your carrier VM.