As a kid I was taught that I should be able to put one foot down flat while sitting in the saddle. I think I operated under that assumption as an adult until I was told otherwise. It's probably something that works better with a kid physique.

I find that if my seat is high enough for efficient pedaling, I can't actually sit in the saddle and put a foot down with the bike still upright. The bike will be leaning to the side if my foot is on the ground enough to be stable. Alternately, when I'm stopped I'll stand in front of the seat with one foot on a pedal, and as I push the pedal to start up again I'll lift myself into the seat as it moves forwards.