Been away from this thread for a few days - about the curving of the cut at the end (or beginning). To minimize this, the depth of your cut must be just deep enough to penetrate your backing board (which really should be the same kind of board as the matt. (If you don't use a backing board, letting the blade tip run into the "trough" in the board, you run the risk of ragged edge cuts.)
When I get a new pack of board, the first thing I do is mess around with the depth set screw so that the blade cuts just deep enough to go through. The reason this cutter makes nicer cuts and over-run cuts than the Dexter is that the blade is thinner. If the blade is running too deep, it can curve (in the middle of the cut too, you just might not see it because the right angle of the side isn't nearby).
The next thing is to be sure that the board is sitting on a stable (very strong, or stiff) table top. If the board is allowed to bow (especially on very long cuts), the blade may not cut deep enough, as the matt and backing "dip" down during the cut, but the length of the cutting tool won't allow it to bend. For this reason, some folks set the depth deeper than necessary to accommodate the bowing, as there is no bigger PITA than finishing an incomplete cut on a large matt.
All of this comes from doing a lot of matts and several conversations with Logan customer service over the years.