There's lots to consider for balancing things out to maximize the "team effort", which is a system. The more you do RIGHT, the better chances you have of attaining a quality effort, and great results.
Sure, you can get buy, and probabaly be just fine doing mix match. And, yes, the rears arent' as critical to the peformance of the system, as what's happening up front...you absolutely MUST match the front 3 speakers to produce a believeable, coherent soundstage with unifor tonality there. And this is the bulk of your pressentation.
Heck, I use two speakers, and sometimes not even a sub to watch on my big screen/receiver setup these days. But then I'm a bit lazy with all of this right now, and don't really care so much. I know..sad isn't it.
Still, if you want to add another properly balanced ingreadient to your mix, and keep the quality level of the system up, and be ambitious about it, you really should get the matching speakers ALL AROUND in the system, to maintain coherence and integrity, and so your soundstage has the potential to be transparant, and so you can just dissapear into the sound, and the gear gets out of the way of the playback chain.
What often happens is people end up listening to a collection of "gear", with limited fidelity potential and realism to the perforance of the audio. It can just end up being sound coming from different directions...and not really great sound, at that.
Think of it as taking the time to carefully spice a dish your cooking. Yes, you can be happy with just "ball parking" it. But heck, why not just throw anything in there, if better quality can't be had matching everything in the first place? Cause you're just not gunna end up with anything SPECIAL from the sytem, otherwise. Nothign you can't get from a better home theater in a box effort, if you just throw it together, really.
Just some thoughts