Tubes: unmatched Quads or Pairs

If you own an amp that has individual bias controls, what is the worse thing that can happen if you install a mismatched tube OR say 1 Svetlana with 3 Sovteks?

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I have such an amp.(s) CJ 5's. With the abundance of tubes, here at this sight /Also since we're not talking expensive "nos"s/ why would one want to mix & match? I would say no harm for a short period of time (less than a week). Amps are expensive to work on / why take a risk?--Not to mention the affect on the sonic integrity.Keep spares on hand that are the same.
The reason why I ask is that my amps use 8 KT88s per side. I purchased 8 matched quads, that I hoped would serve me for a long period of time, I have to now dip into the 2nd set of 16 to replace 3 that look like there are not doing well. I have an odd set of 3 of another ilk, that if used would keep the 2nd set intact.
I had a couple of tubes in my VTL 185s get fried from a near lightning strike. the folks from VTL sent me two new tubes, one for each amp, with no concern for matching. Stuffed them in, did a re-bias and they sound great, bias has remained stable for about 3 mo w/ no problems. I think the matched set thing may be overrated.