RAW offers highest possible quality, theoreticly. I very much doubt if anyone can see the difference whithout side by side comparing though.... in RAW images you can change white balance (and other parameters like contrast, sharpness) later, and you can easier/better correct exposure faults. If you're sure your exposure, WB and parameters are dead on, there's no problem using jpg if you use the highes quality setting. Might save time in post processing.
A jpg degrades in quality every time it's saved. (open -> change -> save). Therefore it might be a good idea to convert all jpg's to TIFF for processing. Depending on your client, you deliver him/her photo's as jpg or Tiff, in desired resolution / colour profile. I would defenitely crop the pics to the desired cropping, not to a specific size. Always keep a maximum resolution or RAW for backup. (If the client wants a 4x6" now, doesn't mean he won't want a 10x20" later...
I'm going to assume you're not new at photograpy, just digital 