I generally listen to music in 2 channel mode, but even with 2 channel material that is matrix'ed to surround, it hasn't sounded awkward at all. It's been a while, but I recall hearing mostly echoes, the crowd cheering, and misc other things from the rear.
I have Paradigm Monitor 7 floorstanders up front, with a Paradigm CC-270 center (a step down from the Monitors), and Boston Acoustics HD-8 bookshelves in the rear. Both 'Digms and Bostons are fairly neutral sounding and both have metallic tweeters (titanium for the 'digms, and platinum for the HD-8s IIRC), so their sound is not vastly different, but they are different. All are typical directional type. To be honest, I notice a bigger difference between the CC-270 and Monitors 7s than anything with the Bostons. I also don't have a lot of 5.1 music, being limited to 3 or 4 concert DVDs and 5 tracks of 5.1 audio from the Blue Man Group Complex tour concert DVD. The effect from the center could be a result of it being a matrixed center in most cases and I don't recall noticing that difference with the 5.1 material.
With that said, I do have tentative plans to replace the Bostons with some kind of bipolar/omni directional speakers in the future. As it is, they're a little to easy to pinpoint with your ears at times (though not often), even with them behind you. I don't think the surround tracks are meant to be directional in most cases, if at all. But I've also had limited experience with omni directional speakers, so I don't know what changes or effects to expect.
I'm not sure about Klipsch and Polk, but from what little I know, I believe the Polks might be a little more laid back sounding, since the Klispchs have horn tweeters. Mixing brands has worked out great for me, but it just worked out that way, having bought the HD-8s years before (for a bargain I might add).