I always custom white balance when I find my grey balance drifts out which is about once a month. It's easier to detect that as I only shoot under a certain artificial lighting.
In fact when I Custom white balance I always do it twice. Don't know why I find I get better white balance when doing it twice.
depends... indoors sometimes i shoot in flash, den set to flash. if the lighting condition is bad, den do a custom white balance, or if looking for a color effect. if for outdoors, normally set to auto, hard to predict the white balance.
For outside..I will start with Auto Mode in -1 or -2 setting. And shooting in nature scene will set to cloudy and again -1 to -2 setting. Both for sunny days too. They give me pretty nice colours and if it is abit "offish" I will correct them with PS.