I can only speak for the very few times I've been forced to use them. There's space for about three or four cabs outside the station, and a ramshackle taxi office across the road. Personally I wouldn't risk being snookered there, either with or without a prior booking. Others with greater faith in the Lewes transport infrastructure may have other views obviously.
The taxi service in Lewes was absolutely fine. I managed to pick one up immediately when I got to Lewes after the game and then again around half 11 when I wanted to go back to Gatwick.