I'd really like to know your entire layout, the other speakers you are using (crucial), and overall acoustics of your space. Not knowing all of that, my first question would be to know what L/R/C speakers you're using? I'd then match to those. In your case, I'd default to maximizing the main front speaker setup - and integration of the subwoofer (all considering good acoustic fundamental practices) - and strongly consider doing in-ceiling speakers for rears.
I would be doing in-ceiling because the mains are most important in anchoring the soundstage, most of the dynamics, imaging, etc. You really want just some steering and ambience from the rears, regarless. If you put speakers behind you, you'll be having a big gap with transitions from front to back. Overhead is less localizable, you have more flexibilty, and will transition from front to back better than back wall only.
A better option for the money would be matching dipoler speakers mounted above and to the sides of your seating on the ceiling, just like in-ceiling. However, care and caution must be used in mounting on ceiling. So anchor wisely.
Yes, my final is to find MATCHING rears to your mains, and put em on the ceiling. I'd alway recommend here over behind with no sides...better envelopement, better stearing, less localization, more placement flexiblity for best sound, out of the way, etc. (if a little more work in installing)