Generally the shared idea is to go with whatever most of your sources are in. If it's close to a 50:50, then it's a matter of taste.
However I personally don't feel too tied to the original AR so I might as well do 2.35:1 even if the source isn't if I feel that the AMV would work better that way.
Since we're talking about mixing aspect ratio, I think it's appropriate to expand about mixing framerates too.
Wider ARs and 24p framerate have a more cinematic feel (especially true of 2.35:1), which can do good to videos that center on mood/romance/drama/pretty imagery, whereas with a 4:3 and 30p framerate you can convey a more "raw" or TV program feel, which can be more appropriate to comedies, but also works for horror or other genres in which a "live" feel can be a plus (think of something like REC).
So imho, if you're going to be changing some of the sources AR and mixing stuff, be it by mask or what, I think it could be appropriate to just think what you want to do with the video and which feelings should it convey, and try to match that with the tech aspect. Do use resolution and framerate as a tool and a mean to get to a point and don't just see them as a static limit for what you want to show.
But for the love of god, try to avoid upscaling and duping frames as much as possible. Try to use the smallest non-awful AR possibile and always assuemfps to change the playback speed of your sources when you need to modify the framerate. Dupes and removing frame make the video skip like hell, and upscaling by too much makes the sharpness difference painfully obvious with blurry sources mixed with detailed ones. It's the same reason that might make you want to lowpass/blur a tidbit when you downscale HD sources when mixing them with DVD source, since just downscaling the HD to 480p will likely still look sharper and more detailed than the DVD. Inconsistent look is a thing to watch out for when using multiple sources.
If you need to upscale, make it by little amounts (when mixing 4:3 and 16:9, 768x432 is a decent compromise, for example). And if you need to insert dupes or remove frames, make it in a way so that you're doing it by even amounts (ie, to twice or half the framerate), so that the motion will be smoother.