I like to do collages of one piece from several angles, it just keeps the gallery from being cluttered and you can see and compare all the various angles at the same time, instead of having to go back and fourth between the different pages. The only thing is that it should be big enough to see the various views clearly.
I like to use collages for works-in-progress or pics that are just thrown together, for nicer pictures it looks better as single submissions, usually as long as it's only a handful of shots (and not 36 pictures of the same thing... I've seen it done before!)
it's hard to say....If they're done nicely collages can look more interesting than just a single photo, and it's fun to get the overview or whole view of soemthing at once. Yet I usually tend to like the single pictures better because they are usually bigger and it's easier to see the details, without having a large view that is so big when I click on it that I have to keep scrolling back and forth to take it all in - that always makes me feel a bit muddled.