In theory, a wall cutter like the Fletcher 3000 should be very good for this.
In practice, like Cliff, I use the 2100. If I never had to hand-cut another mat, I would keep the mat cutter around for trimming stuff down.
The problem with the wall cutter is that you need at least one side already cut parallel to the poster to rest on the bottom rail. If you rely on the hold-down bar to keep it in place, it <strike>might</strike> will slip.
Cliff, please elaborate on "don't put the poster under the bar." Are you concerned about accidently using the bevel blade to trim or have you had other prolems with using the hold-down bar on the 2100 while trimming?
I fired an employee many years ago who went to trim a mounted studio portrait with the mat cutter and ran a deep cut through the image with the bevel blade. Everyone makes mistakes, but he compounded it by trying to touch up the cut with markers. I never saw it or heard about it, and it actually made it home and onto the customers' wall before they noticed.