I took out our old tile kitchen backsplash and cut out the damaged drywall. I re-installed drywall - basically from about 2 inches above the countertop to 2 inches below the cabinets. I taped and mudded the joints, roughly. If I am installing new ceramic tile, how much time do I need to spend on getting the drywall compound smooth and flat? Am I ok with a few small "ridges" of mud here and there or is that just going to get my mastic trowel caught up. I wouldn't paint the surface now, but since i am covering it with tile and a good 1/4" of adhesive, I don't want to do any unneccessary sanding.

All you need to sand down would be any large bumps or ridges. It doesn't even have to look pretty as long as you can lay the tile flat.