I have all 4 ESC's plugged into the APM using the ESC's factory installed 3 wire connectors. Which I've come to learn means the APM is getting power and ground fed to it from each ESC. But I'm using the 3DR power module to power the APM. Should I cut the + & - wires from the ESCs and leave them as signal only?
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If you have the JP1 jumper removed, no power from your output rail (where your ESCs are plugged in) can get to the APM. So you are fine in that regard.
It would be wise to at least disconnect the power leads (center conductor) from your ESCs. Otherwise they are all connected together on the same buss. If one ESC's voltage level is a little lower than the others it could fight them and unpleasantness occur. I find the easiest way to do the disconnect is take a straight pin and use it to release the wire and male connector out of the three pin servo housing, fold it back on the main wire and wrap it in tape.
APMs also seem to work just fine with ESCs that have only their signal wires connected (Some Pixhawk users have reported issues with this). I have a multi or two where I've done this. Less wires connected to the APM means less chance of vibration transmission. But running the ground does serve as a bit of insurance.
I think JP1 is removed, but I'll double check that when I get home. It sounds like I should definitely remove the (+) from the ESC's so I'm powering that bus with multiple sources. Nothing good can come of that. With JP1 removed, and with the ESC (+) wires removed, will the APM + bus have any power at all?
The notion of multiple grounds also worries me. I have never in 20 years of tinkering with electronics and electrical devices of both high and low voltage, found anything good coming from multiple grounds.
If you don't have anything plugged into the bus that needs power, like servos or strobes, then there's no need to hook up any +5 volts to it so you can disconnect all your ESC 5 volts leads. (Unless you are using the new and somewhat strange Castle Multirotor ESCs, which require 5 volts to power the ESC!). And you are correct to have some concern about multiple ground paths. Since the ESCs are already hooked directly to ground, they already should have that reference. In almost all cases where you don't need any power from your ESCs, you can connect only a single signal wire from each ESC and it will work just fine. Unless you have a Pixhawk, then it may or may not work without individual grounds.