I have only used "winkflash.com" and "costco.com" so do not know if my answer apply to the professional level print shops ...
All my photos straight from camera (RAW to JPEG) can be printed 4x6 or 5x7. I shoot in the best quality allowed by camera.
I always download the max size instead of the "fast download" offered by CostCo. This ensures all my shots will work with their largest print (20x30 I think). I use a Canon 30D (I think 8MP).
If you are cropping, Photoshop Elements allow me to crop based on dimensions (4x6, 5x7, 12x14 probably allow strange dimensions as well). When I want to print some "odd" dimensions (12x14, 20x24), I always crop before sending off to print.
Hope this answers your question.