In the case of your amp, all four KT88's are biased at the same time. If they are not matched for current draw, you may have one or two drawing more current than the others. While the total may be correct, the individual tubes will be running at different levels. Depending on how far out they are, it could be audible.
The EL84's don't need to be matched, and for equal gain, the 6922's should be matched. That said, I've never seen preamp tubes so far out to make much of a difference unless one was defective.