How important for surrounds to match fronts?

I have done it both ways, and having all the same brand and family of models designed to work together a big difference in sound. I find on both 5.1 Audio (SACD/DVD-A) and movies especially noticable on DTS soundtracks. As mentioned is it a must have? No! But is it an avenue to maximizing the sound track reproduction, in my opinion and experience, Yes! Especially on movies where the sound shifts from front to rear or visa-versa, matching speaker voice creates a seamless transition. My advice is, if you can afford it, do it. If budget doesn't permit then try to voice match as closely as possible.

Take a look at this article, especially the last comment. All identical speakers is the absolute best for a multi-channel system, theater or audio, but difficult to use without a dedicated room. The best alternative is a timbre matched system using center and surrounds designed for the mains.

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.

This was true many years ago with Dolby Pro-Logic, however, today all channels are discrete, uncompressed, and full bandwidth. A sound may move through the room in anyway imaginable, or be fixed at any place in the room, therefore any two or more speakers may be used to create the sound. Looking at the picture in Matching Front and Surround Speakers, it is obvious that the jet would sound exactly the same, front or rear in the room, with all identical speakers. Using timbre matched rears should be extremely close, but still not exactly the same, however, mismatched rears will have a definite difference in the sound. This is easily heard by running the test tone around the room. For all identical speakers, the tone sounds exactly the same in every channel, and with timbre matched, it should be very close, but likely a slight difference. For mismatched surrounds, the difference will be much more. Matching the rears with the fronts will give you a better balance to the sound throughout the room.

I have Aerial 7b and CC3b up front, Swans (much less expensive) in rear. I only use for movies not music in this set up. I think that having matched rears is less important for home theater, and also less important unless the speakers are quite exactly placed.

I've done it both ways and gotten good results both ways.

My listening room does double duty as a home recording studio. Since I use a pre-pro in my 2 channel rig, I'm able to quickly go surround if needed for an audio-video session. I have a small Marantz projector and use 3 of the 4 powered M-Audio monitors that are usually dedicated to the recording studio or connected to a remote Sonos zone as surrounds and a center. These M-Audio monitors are both less than optimal for surround duty (dispersion) and very different from my main speakers - usually Ohm 100s, sometimes MMGs, occasionally Verity P/Es. In all cases, the results are satisfying to me.

YMMV

It may or may not make a difference. I had Focals as surrounds paired with my Revel fronts. There was a difference. One wasn't necessarily "better" than the other but it was different. When I replaced the Focals with Revel S30 surrounds, I only then appreciated what the full voice matching did for the surround experience. It was truly seamless.