I haven't painted cabinets, but I would say to definitely clean them before sanding. If you want the paint to stick, surface prep is the most important thing. Also the least fun to do, but its important. Scrub them well, rinse well and dry with clean rags. Sand, then wipe down with a tack rag, then a clean rag with something like denatured alcohol? Not sure about that part, I used denatured alcohol to prep my cabinets for refacing veneer, someone who's painted cabinets could tell you if that is correct for paint prep. THe denatured alcohol removes any oils or residue. Let dry well, then paint. Surface prep is tedious for sure, but so important if you want paint to adhere well. I would also suggest a good primer, then a good paint. I haven't stated painting my bath cabinets yet, but I have read on this site that BM cabinet coat is a good choice.