Thanks for the response. The systems I'm looking at are kind of low-budget for now, as they won't be getting intensive use (I don't gig too terribly often), but when I do, I've found that I'm awfully prone to want to roam. So I'm not too terribly worried about running into someone else's system interference, but rather my own.
The instrument transmitter is a Channel A, and the microphone comes in Channel A and E, but the E costs $20 more than the A. Looking at the manual (it's all the same manufacturer) it says:
"Available on selected frequencies in the VHF high band (171-216 MHz) for interference-free, long-range performance"
So if I'm buying two receiver/transmitter sets of the same brand (and practically the same model), and they will likely be sitting right next to each other, I should probably go the extra $20 and make sure I get them on different channels, right?