I'd suggest that the stretch of nylon is somewhat related to the rated strength. If you're using super strong (2" wide webbing rated to 10,000 lbs load) then there's not going to any significant stretch to worry about. But 1" rated to 300-400 lbs nylon might stretch enough to be an issue at least sometimes.
Of course in between will be in between...
I recently went on a 3 nite trip using SlapStaps for the first time. The stretch was significant all three nites. I thought by the third nite I would have stretched it out, but by the next morning I ended up close to the ground. I had devised a biner with a loop on it to take out the space between the standard loops in the SlapStraps to make sure I had it as tight as possible when I got in each nite. I now think I will convert over to the buckles. I have the HH Explorer Delux and will have to figure out how to do the large rope (1/2"?) with these buckles, but that is another issue.