Yeah, I usually just keep my dislike or annoyance with someone to myself. This is because I know my opinion of them is open to change, and it's not really worth making enemies with someone or making them feel bad because at one given moment in time I feel like I don't like them. If I don't like them to a high enough degree, then I can just avoid spending a lot of time with them. Unless the person is actively antagonizing me, in which case I can be provoked into antagonizing back. Sometimes, if it's bad enough, I can get nasty.
Even then, however, I don't really think "oh God, I hate this person for all eternity." This has gotten on some people's nerves because they wonder why my value judgments are so inconsistent.
For instance, there is this one girl who has an annoying and sort of grating personality, and has a history of not being nice to me and a couple of my friends. One day we were talking about something she was doing during class (she was making weird facial expressions), and my friends all ascribed the worst possible motivations and intentions to her behavior. I started arguing that we don't even know whether that's the case or not, and they were all like, "Orangey, you know she's a jerk. Remember how she treated you and so and so that one time? How can you not think that her motivations are bad?"