First, the blending. Even though you order say 65% density, the density will still vary from piece to piece. Additionally the actual thickness of the hair shaft can vary from piece to piece. Because of this sometimes pieces need to be thinned with thinning shears to get a good blend. Sometimes they need to be thinned a little, sometimes a lot.

Second, ventilation. ventilation isn't always perfect. Sometimes you get some spots where the ventilation doesn't follow the rest of the piece. This can make the cut a little more difficult.

If you have the comfort level, and the money if you mess up the piece, to do your own cut in your own pieces then I say go for it. I've seen some amazing self cut in pictures on this forum. So it can certainly be done.

If I could cut my piece exactly like I want it, at this point I would still go to a professional. I'm a fulltime and a half student. I work at an unpaid internship. I don't have the money to throw away on extra pieces if I screw it up. Once I finish my doctoral then I will most likely try my hand at cutting a piece or two. But even then I wouldn't attempt it without at least one professionally cut back up piece. Just in case.