Great! So glad you got it all set up and running. I completely understand the "do we really need this" line, heard it many times myself in the planning and install stages. Anyway, an SPL is one of the best things you can buy to get it all dialed in properly. Definite improvement in quality over by ear.

As for something to audition the rears, may I suggest you buy/rent/borrow a copy of Star Wars Episode 1. Excellent use of surrounds there from start to finish, including the rear center.

Anyway, congratulations and hope you enjoy it.