An old mate of mine used to regularly do 4 or 5 clean top ropes of a sport route before he would go for the redpoint. He just hated taking leader falls, and was incredibly fit.
Yeah only go on toprope once you know the moves when it is wet and there is a danger of you pinging off unpredictably. That said, there are ppl out there who are naturally scared of leading - doesn't make u a bad person and these ppl should not be chastised for their yellow bellied-ness. I once spoke to Steve Roberts on the catwalk and we touched on the reason why he always went on TR. He said he nearly died in an horrendous fall in the Moelwyns in his twenties when all his gear ripped out. After that he could only bring himself to go on lead on hard routes after 5 clean TR's and extending all the draws to be in exactly his ideal position on the route. Its why he never lead Cry Freedom after TR'ing it 3 times.
Personally I've never done a route in a oner on the top-rope before going on the sharp end.