With a tool so tough as Dewalt's DW718 12" double-bevel sliding compound miter saw, it is no mystery that the sucker is going to get a little kicked around. Of course, the regular abuses of jobsite life will wear on the tool and eventually it's going to require a bit of tinkering on your behalf. Don't fret though, said tinkering is generally simple with the right tools and a bit of patience.
It may surprise you to learn that the leading cause of inaccurate miter saw cuts is actually a misaligned or warped fence. As many awful things like kickback and sloppy cutting can result from a misaligned or otherwise off-kilter rip fence, your fence alignment and the fence alignment process are important parts of miter sawing and miter saw ownership; despite the seemingly elementary process of this procedure, it can make or break your cutting results. Additionally, because Dewalt's DW718 packs such a serious amount of punch, ensuring that your cuts are accurate and your materials are safe from error or kickback is all the more important. Be precise and be cautious.
Dewalt Reciprocating Saw
So, to get down to business, using a framing square or straight-edge that is at least as long as your fence, press a piece of ultra thin paper (i.e. tracing paper or telephone book paper) against the fence (or straight-edge). The fence and square should sandwich the paper. Now, tug softly at the top of the paper. It should be tightly stuck between the fence and the square at all points from one end of the fence to the other. If at any point you can remove the paper from said contraption, you have a bow, a sway, or an imperfection in the fence that needs to be corrected.
To adjust the DW718's one piece fence, loosen the screws closest to the imperfection and remembering to not over-correct use a pry bar to gently force that bent fence straight. Maintain pressure on the pry bar and tighten the screws. Recheck this space to be sure the bow has completely straightened. Repeat the process until the paper can not be removed from any point on the fence. Additionally, and as with almost all things in life, this procedure is generally easier with four hands; if someone can help you, gratefully accept their offer and hold and push and pull and test and measure until your fence is perfectly straight. Additionally, and I reiterate, please be careful as you correct your fence; move it only in very small increments to avoid over-correcting any imperfections.
If you discover you need an extra hand aligning the fence on your DW718 miter saw, simply take the saw to a factory authorized Dewalt service center for adjustment.