While that can have an impact, the more important question to ask is, "Are they on the same phase?" When multiple pieces of interconnected gear are plugged into outlet circuits with different phases, interchassis currents will occur. Often, people assume that the resulting hum is from a "ground loop", and they start lifting grounds until they finally find the one that stops the hum. By this point, the whole system is usually floating, which makes for a shock hazard.
When you do have multiple pieces of gear plugged into different outlet circuits, make sure that they are all on the same phase. If you have 240v single phase service, this would be every other breaker. If you have 3-phase service, this would be every third breaker. If you have fuses, or some older distribution system, then who knows.
One thing that you can do is to run an extension cord from one place where gear is plugged in to wherever else you'll have gear plugged in. Use a voltmeter and test between the hot leads of the extension cord and the wall outlet. If your outlets are wired correctly, this would be the smaller of the two slots. If you measure 0 volts, then the outlets are the same phase. If you measure 208 or 240 volts, then the outlets are on differing phases. Either find another outlet with matching phase, or if you know what you are doing, swap some wires in the distribution panel.
