There are some clear reasons to decline a review request, such as conflict-of-interests or not enough free time (e.g., going on a vacation..). But what if you are just not interested in the paper you got (i.e., it is loosely related to what you do, but not entirely irrelevant)?
On which occasions is it appropriate to decline a review request? Does it matter if the review is for conference vs journal?
There are a few clear reasons to decline a review request, although in complete honesty, I've never actually declined to review a paper yet, so these are all at least "in theory" for me. Some of them are one's you've mentioned, but there are some others:
- Conflict of interest. This one's pretty clear, though with the way some reviews are handled - based on recommendations, closely related expertise, etc. what actually constitutes a conflict of interest can get a little vague.
- Lack of time. This is one that people seem to ignore or discount, but it's a big one. If you can't give a paper the attention it deserves, or your review is going to be late (predictably, not because of unforeseen things), then you should probably decline to review it. You're not doing you, the authors, or the editor any favors by making them chase you down for months to get a review.
- Lack of expertise. If you read the methods section of a paper and your primary thought is "Huh?" not because the paper is unclear, but because its far afield from your expertise, I'd strongly consider contacting the editor for advice and asking not to be a reviewer.
I wouldn't necessarily not be a reviewer due to a failure to find the paper sufficiently interesting. If your expertise is indeed appropriate and the work is not of sufficiently compelling interest, that is a review finding all its own. Additionally, one would hope that you can evaluate the scientific merit of things that - while you are capable of understanding - you might not find directly interesting.
Whether or not it matters if its a conference or a paper likely depends on your field (how important are conferences?) and the particular conference or paper. For example, I might make a special effort to "find the time" for a journal I submit to (or would like to submit to) or a conference I frequently attend, but might not for a journal or conference I've never heard of.