Here's the general scoop: If your antennas are too close to each other, you can screw up your VSWR. The antennas will interact with one another. Maybe it's ok...maybe it's not. If you look around you may find references to output amp damage occuring but I SUSPECT that this is occuring for the same reason lots of output stage radio damage used to occur...high VSWR. Without getting technical, hi SWR's beat up on the output stages of your radio.
Some newer radios protect themselves against damage (so that's probably OK), but you could hurt the performance of the antenna (there are a million ways to hurt your antenna's performance, though...this is just one of them). It doesn't nescessarily mean that you will hurt the performance and also consider that you probably don't need max performance. Somewhere in there is a "good enough".
The VSWR is an excellent measure of your antenna's performance (i.e. all the energy you're throwing into the antenna is radiating as opposed to "bouncing" back and heating up your radio). Regardless of how you mount your antenna's, it's a good idea in general to check it out with a meter and be prepared to tweak. I'm guessing that lots of people will disagree with this, and that lots of planes are flying that have never seen a VSWR meter and have absolutely no radio problems whatsoever. If you're at all concerned about your installation, though, there's an easy way to check it with the meter.