The several sound engineers that I’ve worked with use the 180 flop switch mostly to help minimize feedback in live settings and to minimize bleed through from instrument, vocal and extensively on drum mics. Because phase summing is different for every environment, most powered subwoofers for home theater use have an adjustable phase control. So, every pedal can be considered a different environment.
As Bongo has pointed out, phase issues are tricky to deal with when you have multiple pedals in parallel. Any pedal that has its signal AC coupled will have phase rotation, that’s 99.99% of all pedals manufactured. Then there is inverting and non-inverting amplifiers that the signal passes through. Typical tone controls on each pedal also change the phase of the signal at different frequencies. Even if you had two identical pedals in parallel, having different positions of the tone controls will create additions and subtractions of the signal when mixed together. Will this be a bad thing? Not necessarily, the result might give you a boost or cut in the frequencies that you find very pleasing. Or the result can be so small for you to even notice a difference.
Now in this typical scenario, I have two pedals activated in a loop and everything is working well. Then I deactivate only one of them for part of the song, my sound suddenly falls short of what I had expected due to the additions and subtractions of the signals. A way to help this would be to rotate the phase of signals in the loop so that they minimize the problem. Sometimes a 180 deg. flip will do it… (invert signal) OK, now all is cool until I kick that pedal back on. The 180 invert switch has solved the problem for only one state of the pedals in the loop. A lot of times the best phase rotation is somewhere between 0 and 180. This way you can try to minimize the problem for both states of the pedals. As you add more and more loops things can get really frustrating to control.
