How important for surrounds to match fronts?

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.

I think that matching all speakers and timber matching is critical to the home theatre experience. I do think that when creating a home theatre like the movie experience the only way is with horn drivers-like the theaters have. The heart pounding dynamics are only created with a full horn loaded speaker. Plus subwoofers to add those last octaves. Take a look at Casta Acoustics and see what I am speaking of. Custom horns are simply amazing. You will not need another speaker.........

If you are focused on non matched speakers during a movie then turn it off.....its a bad movie. Speakers within reason will be fine.

I personally care nothing for multi music surround, but rather only movie surround. That said, by far, I find that effective diffuse dipolar or similar type surrounds - which are EQd out from the main processor, to match the timber n response of the mains - which envelope the listening area, are of more importance than matching or identical speakers all around. But to each his own. (Surrounds at most movie theaters not same as fronts either!?)
The main fronts n center are BY FAR the most important for anchoring the front soundstage, n carying the content, IMO. The surrounds are mostly for carrying secondary ambiance cues,and should be placed so as to not be localizeable, create a large diffuse side and rear soundstage, and for "fill" (as a priority order, that is -minimumally speaking). Thankfully, it's also most impractical to place larger, even full range, direct firing traditional loud speakers up on rear walls n ceilings, anyway.. Not to mention decor eyesore!
Yes, mostly match main fronts, n work on system set up n integration, proper crossover response n placement, and EQing, I say. let the diehard audiophile music only lovers worry about 5 or more matching speaks! The wife or girlfriend will never love it anyway, otherwise.

My opinion, if you want to achieve the movie experience like/better than the theatres in your home in a medium to large space to work with, you will need horns and the less crossover and calibration the better as it is all filtering holding back dynamics and natural quality. If speakers are chosen correctly, crossovers and calibration will be minimal. Identical speakers/drivers are best for timber matching across the range. Dipole speakers beside or behind listening position or both are beneficial to surround duty. Carpet or cork is good for flooring and paneling behind front speakers on wall and in back corners work extremely well. A suitable sub(s) are the icing on the system for low bass duties. Placement is critical with sub(s) as to smoothen out the low end. In the end if this achieved, everyone will come to your house for movies instead of paying and waiting at the local theatre. Enjoy and cheers!!