Keeping the teams even is very important to me. It keeps the games competitive, and leads to less frustration and better in-game conduct. Nothing gets people more ticked off than uneven teams.
If I see that the teams are uneven, I usually wait until the end of the round to see if anybody joins/leaves. If the teams are still not evened up, then I will switch.
As far as evening up for skill, that often times is hard to judge. I rarely, if ever, even teams based on skill. I usually just wait for the game to end and the next game teams are typically mixed a little better.
The most important thing is to keep an eye on the teams during a match. If the game is competitive, losing just two members of your team can make a HUGE difference. Also, on GC's server, it is important to count bots as well--sometimes they get way off balance, and nobody notices because the the team sizes are the same.
Honestly though, I have rarely run into a problem on GC's server and that is why it is one of my favorites. I know that Mountain_Dew always makes an effort to keep the teams balanced if they are horrendously stacked one way or the other, and I have seen Penty and other GC members do that as well.
Anyways, just my two cents.
Game on my friends!
-Kor