Okay, it's far from romantic but I think you should ask out girls as soon as you realize "you click" with them. Don't wait until you have a hopeless crush.
1) Girl only respect guys with guts/confidence. This is where we get the "derp derp girls like assholes" stereotype. And girls are better at reading body language, they will realize if you like them.
2) The longer you know each other the more chance there is of being friendzoned.
3) You don't have much of an emotional investment, you can bounce back and ask a different girl the next day.