No, there's no reason to be selfish. But there is reason to be cautious:
1. Make sure you WOULD NOT attend the school you are withdrawing from regardless of circumstances (scholarship $$$, etc.). At this point, though, most schools will just be awarding need-based aid, so it *shouldn't* vary much from school to school.
2. Make sure you will be able to pay for the school/schools you are considering. Likely you'll be able to, but bad credit history may mean otherwise.
3. Make sure the school/schools you are still considering are cool with any "changes" that may have happend since submitting your application. Fail a class last/this semester? Let them know. Drop a major, switch degrees, have a question about a prereq? Let them know. Commit a crime? Let them know. Once you know you won't be unceremoniously booted in August, go ahead and withdraw from others.
4. Look over the school once more, just to be sure. You've invested likely over $200 in this school, make sure you are sure. Gut feelings or a careful pro/con analysis of location, curriculum, students, etc. both work, just make sure they're genuine.
I'm not trying to tell you not to withdraw. I've withdrawn from three schools already, and will likely be dropping more once financial aid packages are done. I'm just suggesting you cover all your bases. If you have, it is polite and professional to withdraw, other students want a place to go next year as early as possible.
