My several cents, serious collectors prefer unmounted prints. Having said that, I've noticed George Tice still dry mounts his gelatin silver prints. I used to mount and float the trimmed print in a 4-ply matte. Now I don't mount, and cut the window to the image size, leaving the white borders intact and under the matte. I came to this conclusion after attending a talk about archival consideration years ago. My work with the Estate of Andre Kertesz has confirmed this. For you it may be an esthetic consideration, but practically, prints tend to yellow form the edges over time. Trimming the edges means the yellowing will start in the image area. In the end, however, I say do what you like, just be sure to consider your choices carefully (as apparently you are doing).
Also I want to reiterate what Vaughn said, I've never had tissue bleed around the edges.
One more thing. In putting squares in vertical mattes. there's an interesting formula for that. Here is a jpeg of the procedure that was given to me a while back. I'd be glad to explain it if anyone is interested.