Hi there -- thanks for the great question!
I have slightly mixed feelings about this now -- I used to say that you should cheat like crazy and all that mattered, at the end of the day, is how your animation looks from the camera's point of view. However, over time, I've found that the more correct I make my animation in 3D space, the better and more correct it looks from the camera. An added benefit is that it tends to be much easier to change/edit when the time comes or when changes are requested if I've animated it "properly" to begin with, and if you have any kind of cloth simulation or anything like that that will be done on top of your animation, it's going to look a lot better if you haven't cheated things.
So, nowadays I would recommend that you try to make your animation work in 3D space, BUT always keeping an eye on how it's going to look from the camera's point of view. Some things, like arcs, for example, should be especially considered specifically from the camera's point of view. However, I would also say that if you're having trouble getting something to work in 3D space, but it looks great from the camera, well -- you might as well just leave it alone and deal with it later, if/when you get a request from the Director to edit it, because no matter all the pros and cons, eventually the camera's point of view will be the only thing that really matters.
So yeah, there are lots of pros and cons either way, but these days I try to cheat as little as possible, though I do still cheat *something* in every shot, even if it's something "small" like specific arcs or something, and I *do* still have the occasional shot where I've cheated the whole thing like crazy in order to get the composition or staging that the shot needs...