You will get both answers here but I will always cut it down the back. Harder to get an accurate measurement but much less stitching and easier for your taxidermist to stretch. Either way works just fine.
I would ask the taxidermist you plan on using. I prefer rug cut. What takes extra time to sew, saves time in prepping and fitting for me. But that's just me. Everyone does it different, and there's no right or wrong.
+1 on checking with your taxidermist. Mine is different than most and absolutely hates dorsal cuts. 90% of the others want dorsal cut. I know exactly how Frank's wants shoulder mounts, lifesize, etc and that is how I do it.