It's not that I have to choose it manually the first time which bothers me, but that TextMate FORGETS the settings sometimes. So files that have been already saved in the correct encoding are misinterpreted by TextMate to use UTF8.
Also, the other part of the problem seems to be that TextMate assumes that UTF8 is used when it's not sure which encoding the file uses instead of respecting the settings in the prefs.
Max
PS This is just a minor, but annoying bug of mine, I'm a big fan of TextMate :-)