Used a Dexter for several years and did quite well. Still use it to cut mat board down to size. Logan is much much nicer.
Straight corners -- start your cut by inserting the blade partially into the matboard an inch or so early while moving the cutter, then press down the rest of the way at your starting mark.
I found it was the initial pushing of the blade into the matboard that would deform the blade -- then as one starts the cut the blade would straighten out and give one a curved start. By starting the cut early while drawing the cutter forward keeps the blade from deforming. Works with 4 and 8 ply.
With 8-ply I found that using multiple passes (usually 3), working the blade deeper into the board each time works the easiest. I cut a lot of 8 ply.
PS -- I did not have money for beer in college, so I put signs up in the Art Department..."Will cut window mats for beer". An average job might cost a 4-pack of Guinness (no cheap American crap! This was before the micro-brews.) One while doing the job, the rest for later.