Any professional will tell you emphatically "yes!", most likely, but for MYOG, it really depends on what you're making. If we're talking sil, and you're making a tarp, then it can definitely affect strength and the ability to get a good pitch.
If you're making a quilt, or a stuff sack, or anything that's not going to be highly loaded, or need a ton of strength, then the answer is no, it doesn't matter.
On a shelter with a lot of complex panels that are supported? I wouldn't think it'll matter *too* much, but it's going to depend on a panel by panel basis. On an A-frame, I'd say yeah, it's going to affect the pitch a lot. Stretch is a major factor of the sag issues. Although just as big a factor could be the use of polyester thread for a ridgeline seam vs nylon. Nylon will stretch with the seam allowing a better pitch, poly wont.