It is 100% valid to tell someone how you feel when you feel it. You can phrase it nicely or be blunt, but there's a respect to be gained by being up front and courageous enough to say what you feel. I think you draw the line at how you intend to act on those feelings. Don't kiss-rape people just because you feel an emotion. That part should probably be something you want to take time with. But how you feel? Let it out, man. Save everyone some time.
If you're honest and he / she takes off, you learn sooner rather than later that they aren't what you want. Big win for you: you get to move on. Prolonging pain just makes it worse. And maybe you won't hear no, you know? After all, you can't win the lottery if you don't buy a ticket.