Hi Bruce,
For a 5.1 setup, the proper terminology is "side surrounds". The "rears" are only added when you have 6.1 or 7.1. As John mentioned, the best position is 2-3' above ear level, or higher. Mine are 7ft off of the ground and they work great.
Since you have a 21ft long room, why not consider running wiring for a possible future 6.1 or 7.1 setup, wire is cheap? In my opinion using Dolby PLIIx modes on receivers is great. Also, with the new HD formats for Bluray, it might be something to think about.
If you have 6ft of clearance from the back wall, I don't see why you could not go 6.1 or 7.1. My rear Qs8 speakers are only 2ft behind my rear row of seats, and probably 7ft from the front row.
