If you are using a DSLR that shoots in the 4x6 ratio, then posing, composing & framing in-camera (and shooting jpeg

means no cropping necessary for 4x6 prints (or 6x9, 8x12, 12x18). Prints in the 5x7 & 8x10 size usually means cropping before printing, which means leaving space around your subject(s) in order to have room to crop... and more computer/pp time.
We do onsite printing for some special events & even an occasional wedding. It is a pain (I would rather take orders to deliver later) and we settled on 4x6 because there is a large choice of lots of printers available, from dye-sub to inkjet.
If you do a lot of printing onsite, then the 5x7 industrial grade dye-sub printers might be the way to go. But we just don't do the volume to justify tying up that much capital in a couple of them (my business plan requires backups for everything!).
We use a bunch of the older style Epson Picture Mate inkjets (
About 18 months ago I bought a bunch of 64-256 mb CF cards just for this purpose. Works great... not as fast as a highspeed dye-sub, but I have backup equipment that is easy to use and they use fairly commonly found supplies. The last couple of Picturemate printers (old style pigment) I bought brand new were like $40 delivered and included ink cartridges and paper. I am thinking about looking for a couple more.