I agree everything makes a small difference. I usually leave back-plates off unless they are slotted, but it's mostly for easy access, not tone.
On my old Carvins I leave them off also because the plate actually protrudes due to their thinner bodies, and I can't stand the feel of a protruding back-plate.
If you have any issues with noisy springs the back being on will just make it echo worse imo.
Same with truss covers. I leave them off for easy access unless they have offset screws like on Ibanez.
Placing the truss screws directly under the strings is just plain stupid imo.