Sides should be no more than .090" thick. .075-.080" should work fine if the sides don't ripple. I'd guess .090" max for the top, less if its hardwood. Same range for the back, less if its a really heavy species like cocobolo or Honduras rosewood. I just thickness sanded the plates for my 2x4 tenor today. Yellow pine (I guess), WAY off the quarter, three-pieces each, .085". But they are really floppy across the grain, really stiff along the grain. If they had been quartersawn I'd have gone to .075, maybe less depending on the feel.
