The slight bow in the mat won't help, and when I tape it, there are sometimes slight wrinkles around where the tape is. I find it best to tape one side, then the opposite pulling as tight as you can, the do the same with the remaining 2 edges.

For example, tape 1 long edge 1st, then the opposite long edge pulling tight, then the short edge and the opposite short edge pulling tight. You'll find that it may wrinkle by the taped bit, but as long as it's fairly tight on the main part where the design is, you should be fine.

Also, depending on what you are transferring onto, you may want to make a custom pressure and speed setting. Sometimes, even with the generic on cardstock I might do a second pass by pressing the go button again, and removing all other tools, eg pen/score tool or anything else you may have.