I guess it works if you block the trem off, if you don't use the trem at all, or even if you just use the trem sparingly, but I'd never go all Van Halen on a FR-style trem without the nut locked if I wanted the guitar to stay in tune, personally.
I guess it depends on how and how much you use your trem, but generally speaking, I wouldn't say locking tuners are a substitute for a locking nut.