As to protocol, there used to be a general rule that it was best to print with an inch of margin minimum on the paper. This gives ample room for hinging and mounting without putting the image area at risk. Handling prints or storing/moving them can lead to damaged edges and such. With wet prints, damaged edges were more common, such as small chips in the gelatin and scrunched corners. And it is most common for stains from chemicals and such to start at the edges since the edges are more exposed to air, packaging materials and such.
As to size of print for a particular mat or frame size, your call.