Having a great sounding room is more important than speaker placement.
Treating the walls and ceiling toward the front end of the room will pay more dividends than speaker placement. Of course, experimenting with speaker placement is is free and room treatment isn't.
To get a great sounding room, you need several things:
• First off, if possible do not use drywall on the walls and ceiling. It's terrible for acoustics. Wood is much better. If you use finish quality plywood, the walls have some give and absorb a good deal of the low bass. A lot of the room nodes in the low bass area don't exist.
• Using some type of treatment on the front half of the room on the walls and ceiling will take most reflections out of the sound of the room. It will improve the imaging immensely.
• I'm assuming that the floor is carpeted so, obviously, that helps.
• Adding something to the rear of the room that diffuses the reflected sound from the front will, with the proper set up, make it seem like you have a surround system even when you don't. In my case, it's some double door closets which I leave partially open. In front of them, I have 3 Boltz media racks with CD's and DVD/Blu-Rays. The combination creates a very diffuse reflection in the rear. It sounds fantastic.
• My theater is in a strangely shaped basement that seems to make the sound even better.
• The room has a drop ceiling with fiberglass insulation between the rafters above. Replacing 12 of the ceiling tiles with GIK Acoustics panels were transformative in improving the rooms sound.
Again, it's not free, but the ROI was off the charts. I was just very lucky with having a good sounding room to start.
Honestly, though, I've created great sounding rooms starting with rooms that don't sound nearly as good from the start. It does, though require more cost to do that.