How important is it to match tubes in a preamp?

not on every preamp. 12au, 12ax are dual triiode input valves that are matched to themselves.

Sonic Frontiers considers matching tubes in their pre-amps important.

I'm not sure about your question, but designers aften design their circuits using the charastic's of specific tubes. As often as not the same pre-amp will have several different types of tubes serving several different purposes. Its not important so long as you keep the same type in each circuit as designed.

What preamp are you talking about? It is more common that the line stage has two of the same tube (one per channel).