warm-up speed before competition

I noticed that when I time myself at a work-out going at a sprint pace I improve my time by a little bit after every such try done back to back with good rest in between, so I was wondering(since I never competed yet) if you try to warm up with a couple of sample race-pace swims right before the competition(i.e. a few minutes before it) so that then you know what time you want to beat and how much faster you want to go.Or would that be a bad idea since obviously you can excess with too much of race-pace swims and be a little tired for a competition swim? My guess is that would help for 50 and 100 but not more than that?How do you do it? P.S. I am still braving up for my first meet, so sorry if some of the questions are stupid or too obvious ...