Surround speakers aren't as critical as the front 3 when it comes to matching. You do want your center channel to have the same sound character (timbre) as your main speakers. Most folks focus the bulk of the budget on the front 3 and if you need to pinch a little to stay on budget do it with the surround speakers.
Now, something I consider more important than surround speakers would be a good subwoofer or 2. Surround speakers are great for movies and add another layer of immersion but there's nothing like the visceral impact of a good sub. Good subwoofage adds a lot to music as well, giving it more depth, weight and texture. If you don't already have subwoofage I would recommend starting there, along with a matching center then the surround speakers.
