stock strings are usually made from Fast flite and they do stretch, some stretch more than others depending on how well they are made.
but usually after 100 shots the string has settled in enough to twist the string up to its proper length and to get everything back in time and to use a tubless peep without having any problems with it rotating out of alignment at least I have never experienced any problems getting a tubless peep to line up on a stock string provided the string is settled in.