Most collectors and galleries buying prints prefer un-mounted (as in dry mountyed)and with a good border aroound the image typical 1.25". If the prints are matted then it's to archival standards.

Whether you present square images in a square frame or rectanle (20x16) is quite a personal choice, there's no right or wrong. My exhibition prints get framed in my 20x16 and larger rectangular frames but I'll often sell individual square prints from 6x6 in square frames (I prefer to print full frame with no cropping).

Ian