It's probably safer to cap late than not at all, although no one's ever done a phase III trial on this.
Think of infection as a game of probabilities. The more time you spend with your naked weiner in her pus filled vagina
the greater your likelihood of you getting whatever she's got. It's not a done deal at first poke.
I once read an ancient study by the U.S. Military where they tested the "wash the cooties off" hypothesis. They took soldiers in 1940-something and got them within 30-60 minutes after BB exposure at several local bordellos (who the hell knows how they managed this!). The military weiners were treated with both a direct injection and a sulfa plaster followed by wrapping them in wax paper for a certain amount of time (hopefully all this was being administered by beautiful nurses? Which would also explain how they got the Joes to show up in the first place . . . ). The end result? All that scrubbing and cleaning had no effect on preventing disease. Now maybe if you scrub your dick in the first 5 minutes it will be different, but who knows? You're a test pilot on this one!