Sometimes people request that a deviation of mine be included in a gallery of theirs, and as those messages don't let you respond with an explanation, I kind of want to make one. My decision making on this has always been very simple:
If the title of the group is directly relevant to the image content then I'll usually say yes. For example, if it's a group dedicated to fanart of a particular character or series, and that's what I've drawn, then the answer is probably yes. If it's just some random art that I've drawn but it's of, say, someone with a dragon, and it's a dragon group then yay! Dragons! Sure!
I will say no if the group title infers attachment to a specific "ship" or "pairing", or otherwise has little to do with the image content. I just don't think a picture of a specific character standing alone should end up in a gallery dedicated to that character and one other, particular character. While I appreciate the gesture, I know that if I were looking at a gallery for a pairing I wouldn't want it full of stand-alone pics, or if I did see one of those and like it, end up searching that artist's gallery for actual pairing shots and getting nothing for my efforts. And even when I do draw certain couples... it's with lighthearted amusement, and I don't want to get tagged into "the ship" as a whole because that simply tends to attract unwanted attention. Groups with titles that are vague to the point where I can't figure out what the group is actually about just by looking at the title... I generally just say no out of uncertainty? I'm just trying to be mindful of where my art ends up, so forgive me if you think I've been too harsh, it could have just been my own initial confusion about "what the hell is this groups thing spamming my inbox nope nope nope" too. Thanks for understanding