yes, the curve will be changed.. it will be deeper. as you know, adding 1.5-2" per side can give you up to a 4" deep curve, if both sides pulled out on full horizontal plane - you can even play with the "tilt" of the curve, my simple directions were for a flat tarp, rectangular. but with a tarp say tapering from 7' at front to 5' in rear, you can tilt the ridgline downward so that the back half of the tarp gets a steeper curve than the front half. this is visible and observable on many of the tapered cat tarps (see pics of tarptent, mld, and spinntwin, et al.)
one thing to note: once i'd sew the ridgline i would then cut the tag fabric within a 1/4 inch and then fold it over, to sew again. zpacks uses this method.. however a felled seam might be stronger - this method worked for me for many tarps and is holding up on my big hammock tarp just fine.