How important for surrounds to match fronts?

I'm sure others will say otherwise, but IMHO it's no big deal if they don't match. I have Sonus Farber fronts, a Monitor Audio center, and Cambridge rear surrounds.

Having surrounds whose timbre closely matches that of the fronts is a pretty big deal, IMHO. The improvement in the transition from front to back is noticeable - the entire soundfield becomes more cohesive and natural.

I used Gallo Ref. 3.1s from my fronts and Gallo Dues for the Center and Surrounds. And I thought it sounded pretty good. But when I swapped out the Dues for Ref. AVs, the improvement was way more than subtle...

-RW-

It's a "nice to have" .... Not a "need to have".

Visually: matching is highly sought after for sure.
Sonically: .... Preferred / Nice to have, but not a critical "gotta have it" moment.

The rears and rear surrounds are a purely "manufactured" "fill-in" signal path sourced from the pre/processor. It's not the same as creating a seamless soundstage across the LF and RF as in two channel critical listening.

For HT, the audio differences in audio / soundtrack performance are significantly reduced ... thats why they make di-pole surround speakers for cramped listening environments without a fatal result .
For the same reasons that's why significantly lesser grade speaker cables are frequently used for the rears and surrounds. You are not going to see rears and surrounds cabled with Nordost Valhallas even though the fronts representing the critical tracks can be.

For multi-channel audio, sure... It's preferred. But ..... Unless it's a rare audio track actually recorded in the rare discrete channel ultra high-Rez PCM 7.1 format (e.g AIX records) wherein the discrete soundtracks place you on stage in the middle of the performers, the differences in audio performance are similarly nil to negligible.

For the 99+% of your audio and soundtrack listening, .... It aint gonna cripple you to select different rears and surrounds.