It's important to have good timbre-matching across the front, but to say that all speakers need to be identical is a little silly. The overwhelming majority of surround systems use a center channel different from the mains (including very upscale brands like MartinLogan).
As long as your speakers are from the same manufacturer, and particularly if the drivers are the same, you'll get good results. Even noticable tonal shifts are unlikely to dampen your enjoyment of a movie or concert DVD.
If you're really concerned, however, you can get single Axiom speakers as well...although placement the becomes problematic.
