I make a string to the length on the bow tag. Yes it is possible for a string to be a different length then what it is marked at but I figure why get into variables that I can't control. Before I make one I measure brace height, axle to axle, poundage, peep height, nock point, etc. After putting a new one on I set most everything up back to what it was. I said most as you will probrably find a small difference in axel to axel and brace height depending on the amount the string stretched. All that said, I'd put a new string on pretty soon after having the one on my bow to my liking. Shoot the new one and make sure it (to you) fits and shoots the same. You shouldn't just have a new one made and figure it will be okay to put on anytime during the hunt and be the same. Now for me I just make a new one up each year and don't worry about differences except for getting the peep set in the right place. A proper (quality) made string out of quality material (I use BCY 450 Plus) is the way to go. I can shoot an entire year with mine and when I replace it it is the exact same length when made. If you find a string maker who takes pride in his work then you shouldn't have to worry.
I also would mark the cam. This way you could tell if your string is stretching and twist the string up to bring that mark back to where it should be.