A tag for a single chip is usually too granular for the tag system. A lot of tags get created because the threshold for adding one is fairly low, but that doesn't mean they need to stick around. I went ahead and removed the tag from the only question that used it, and the tag will eventually be automatically cleaned up. This particular tag is definitely too granular since there are four chips in the family that are very closely related - a tag on only one would exclude the other three. (18F4550, 18F4455, 18F2455, 18F2550)
You already can search for specific parts (try searching for 18f2550 on the main site - it still works without any questions being tagged). The tags for a question shouldn't be the place where you get important information regarding the question (e.g. what chip they are using) - that should already be in the question.