There are photographs and there are photographs!
One might be a 6 x 4 from a machine in Tescos, another might be an expensive hand printed image and another might be an old irreplaceable photo of a deceased family member.
So there are no general rules for mounting photos and consideration does need to be given to conservation/preservation.
Here is a link to a potentially useful article:http://www.wilhelm-research.com/pdf/HW_ ... es_v1a.pdf
It does run to 58 pages though!