[kc8apf]

Two possible issues: First simple: grounding. the two legs may possibly have a different ground potential, or stray DC difference. WHY: you get HUM with that problem.
Second:. because of the 240volt differential which could possibly arise from using both legs in connected equipment.
The only issue with using two legs is if an accident, or short developed. A few odd bits of garage tinker sort of stuff might straight up have issues with the other leg, and inverted AC wiring job and chassis ground to neutral..
All sorts of very minor problems with only one leg, can be major problems if they land in the wrong place on a system using both legs.

Normally nothing bad would happen. It is the 'possibility' of a problem, and the damage to you and the equipment if a 240 volt issue arises.

At least in US residential wiring, all grounds run back to a single bus bar in the panel regardless of the leg they are paired with.  There should be no difference in ground potential.  The two legs may have slightly different voltages due to different loads but that has nothing to do with grounding.

[jea48]

I recently had a local certified electrician install a new SqaureD circuit panel. I spoke with one audio distributor who told me that with two vertical columns of breakers, all my dedicated stereo lines needed to be in the same vertical column (either left or right) to be on the same phase.

When my electrician was here doing the work, I told him this and he disagreed and said this was incorrect with this particular circuit panel. He tried to explain this to me but honestly lost me with his electrician lingo and choice of technical words.

Can anyone out there please confirm if my three dedicated stereo lines are wired correctly and all on the same phase? The three stereo circuits in question are # 27, 28 and 31. Photos of the panel configuration, and SqareD model/information are attached for your review.

Thanks in advance.

They are fed from the same leg, Line.
 L2, ( Line 2)
 

[jea48]

VERY interesting. I wonder why my audio distributor told me this, and why my electrician was reciting "phase A, phase B" about 100 times?

Most residential dwelling units in the US are fed from a single phase transformer with a split phase secondary winding. As others have said there is only 1 phase. A center tap of the single phase winding will become the neutral "The Grounded Conductor", where it is intentionally connected to Earth. From either Hot conductor, of the single phase winding, to the neutral conductor you will measure 120V nominal. And naturally from Hot to Hot 240V nominal. Thus a 120/240V 3 wire power source.
And as others have said the Hot conductors are called Legs, or they are called Line. Usually identified by "L1" and "L2".

The term phase "A" or Phase "B" comes from a multi-phase power system.
The typical Utility power company generators produce 3 phase 4 wire WYE power.
(3 separate windings with one end of each connected together making a Y connection.)
 The 3 Hot output leads  are 120 degrees out of phase with one another and are usually referred to as Phase A, B, and C.

In most commercial and industrial building the incoming Utility power is a 3 phase 4 wire WYE AC power system.  3 Hot conductors with 1 common neutral conductor.

LOL, now it is possible to use 2 of the hot phases, say A & B and the neutral to feed a single phase electrical panel.... In this instance one Hot leg would be called "A" phase and the other "B" phase, because they are infact 2 different phases.  You could have this situation in a multi-story apartment building that has separate electrical panels in each apartment.
In this case the power feeding the electrical panel will be 120/208V nominal.