With certain types of dry mount adhesive (the type which completly melts) the edges can be self trimmed. The proceedure goes like this.
Place the print face down on some kraft paper and cover with adhesive larger then the print,
Place in press between two pieces of relese paper,
Close and lock for 30-60 seconds,
Open and remove assembly,
Quickly seperate the sheet of release paper and the kraft paper, the excess adhesive will cling to the kraft paper leaving the print clinging to the other release paper which can be removed when cool.
Now you have a print with adhesive on the back perfectly trimmed and ready for dry mounting.

Chris