I have no problem with doing it. I usually complete the games before editing them, but that's because usually I get these games quite early and there aren't any editors out there yet to use. (I got BG2 the week it came out and was too busy playing it to worry about using SK.)
I tend to customize my games quite significantly when I've completed them. I playtest - create personal weapons, spells, scenarios, etc. Adds to the replayability. Otherwise it would be gathering dust on a shelf somewhere.
Remember - if you think you're cheating doing it, you're only cheating against yourself, so where's the harm? What's important is that you're enjoying yourself. The exception is when you're playing MP - then you should play fair.
