I agree with Munch on the comfort question.
As for your specific question: being able to flat foot is definately a confidence builder. And being on your tip toes can be dangerous, especially for a new rider. When you say you are on your toes, does that mean that you have the balls of both feet down? If so, then you may be OK. But if you are truly on your toes or on just one foot, you put yourself in a situation where you may not be able to get your foot down when you’re sideways on a slope or you stop and your feet are in dip in the road. You also have no traction of leverage for walking the bike back out of a space.
Another thing to consider is that many bikes can be modified for shorter riders. Search this site for more info on options — some are fairly simple and cheap.