See, I think that you can be friends with girls and then switch them over to relationships later, so long as:

A. you don't let them treat you the same way they treat their other girl friends (aka you're different, and you don't take their shit)
B. you don't listen to her bullshit drama and you make sure she knows it
C. you put distance and time between you two prior if you do end up wanting to fuck her. It allows room for a new relationship and opinion of you to form.

Regardless of relationship or friends, I like to tease girls I'm close with. A lot. And I can definitely see a change in them when I stop teasing for a while and they tend to give me a chance to get a little more intimate. Go back to teasing, and they're again on long term waiting period... almost like a long-term attraction stage.

It can die though if you don't keep contact up somewhat regularly, which I've found can become pretty awkward, as it's begins to feel more like acquaintances than friends.

unfortunately for most of the girls that I'm friends with I no longer have any interest in them sexually, regardless of what I thought of them when I first met them. :P