not very important at all, each tube used in the SP9 uses one section for gain, and one section as a cathode follower.
If you had several tubes that were all poorly balanced, with more gain (all on the same side), then one channel might be maybe 10% louder than the other,
but with the signal running thru 4 triodes on each channel, likely the law of averages is on your side. If you wind up having to constantly put your
balance control to the left or the right, then you aren't very lucky.
Also, the tube tester you are referring to is partially designed to run tubes at various points up to absolute maximum gain.
Guitar amplifiers are purposely made to overdrive and distort the signal, something that is never done in hi-fi audio.