On races without tyre wear use Softs, when it's on, use Hard unless it's upto something like 20 laps, I don't really bother with medium's, I either use soft or hard personally. The locking up is merely technique, shouldn't matter without tyre wear, with it though, I'd try and keep it smooth.
I dunno if it's me but I didnt notice a big difference in longetivity between hard/medium/soft tires.