The tip shapes itself after some playing. If you use a lot of spin and less speed, it will be more round. If you use hard strokes, punch the cueball a lot and rarely go way off center, then you'll more likely will have a flatter tip. If the tip is softer, the tip shapes itself faster.
I noticed that, because I have quite round (about dime) and soft tip. I was selling my previous cue (Bear) recently and my friend tried it for a couple of hours. He shoots very hard and rarely spins the ball. When I got my cue back, the tip was a little bit flatter and a lot harder and I needed to scuff it up, because I was miscueing all the time. So, don't underestimate the power of your shooting style, it has a good effect on the tip. IMHO, if you find youself shaping the tip very often, perhaps you're trying to force the tip to be something else that doesn't necessarily suit your style of play.
Just my two cents.