If you can sit on the saddle and put your foot down flat your saddle is probably too low or you are riding a foot forward design or a bent...

The type of bike you ride will affect how much of a toe you can put down... touring bikes have lower bottom brackets while mountain bikes and some road bikes like old Peugeots and fixed gears have higher bottom brackets.

Have designed and built some bikes with slightly lower bottom brackets to allow a more stable foot position when seated which is especially nice if the bike carries higher loads... this the reason touring bikes have lower bottom brackets as well.