Chris, I mount all my prints on Alpharag Artcare board, so I have no worries about any degrading of the image over time. Take a look at
http://www.superiorarchivalmats.com/ for more information and a test done with Alpharag board. I think you will find it quite interesting.
I have a several Adams and Westons prints that date back to the 30's, and they are drymounted in this manor as well. The prints mounted on quality, acidfree, archival board...are still in perfect condition. However, one of my Weston print that is drymounted on non-archival board has started to "silver" around the edges of the print, giving the surface a metallic looking finish.
I was told by photographer Randy Efros (former Brett Weston assistant) that the paper that I platinum print is made on...will degrade BEFORE the platinum coating! I have never seen a platinum print change colors, fade, silver ect. Platinum prints are one of the most archival prints made!
Anyways, I tack the drymount tissue to the back of the print BEFORE trimming it, so I have never had a problem of tissue showing. Just make sure you use quality mount board (preferably AlphaRag Artcare board). The only problem that I can think of with drymount a platinum print, is that the paper is VERY thick and when someone is handling the print they must be careful not to catch their fingernail on the prints edge.
Ryan McIntosh