tl;dr: I'm skeptical that we need it but I would not likely argue over it, which is just another way of saying "not contentious."
Using our "checklist" here, for me I score xkcd a 7:
- Does it make sense to be an "expert in xkcd"? +1 possibly.
- Does it make sense for a question to be tagged only with xkcd? -1
- Does xkcd have a single, universally-unambiguous meaning? +2
- Is xkcd likely to be used correctly just based on it's name? +2
- Are there "enough" (> 15) but not "too many" (> 10% site-wide) questions that qualify for xkcd? -1: Not at the moment
- Are people like to use xkcd to find questions to answer? +2.
- Are there likely some users (be objective!) who will favorite or ignore xkcd? +1 given the range of topics, favorite maybe, ignore unlikely
- Could xkcd be reasonably used to feed questions to a specialized chat room? -1
- Can xkcd be used to search for questions (for any reason) in a way that keyword searching cannot accomplish? +2 (questions/answers might mention XKCD without actually being about one of the comics)
Based on my short sample of good/bad tags, the "probably good" tags score 8 or higher, and the "obviously good" ones at least 10. 7 is pretty low, meaning I would be skeptical that it will be of much use (primarily: I think it will get applied correctly but never used much).
Having said that, we have much worse tags than this, and 7 is borderline. It's also getting close to the arbitrary 15 question threshold, which would push it up to a 10.
Ideally, I'd say wait to see if we get any more questions and then we can go tag them xkcd but I wouldn't object very loudly if someone wanted to do it now. 1-2 years from now if there still aren't many questions we can always remove it again.