"To" is recommended in the AIM. Some people claim that it can be mixed up with "two," but in the AIM's system of terminology, we're not supposed to be saying "two five hundred" or "two five thousand," it's supposed to be "two thousand five hundred" and "flight level two five zero." The same applies to "for" vs "four," although the AIM is silent on that one.The only thing that really bothers me is using "for" and "to" when reading back altitudes. Apart from that, keep trying to sound cool with your "two point five", just know that you don't