The amount of noticeable change will depend on how many jumps your lines have, and how fast the openings are.
I jump a monarch 155 and it can spank like the girl next door. After about 4-500 jumps on the lines I was getting way too many rough openings and was told stretching the lines might help. After I did that she opened like a dream again (well, as soft as she can open).
At the club I jump at, 7hills, we have a hook screwed into one of the wall support beams. I unhooked the risers and linked them to the hook and was able to pull on the lines with my entire body. The whole process took me about 1.5-2 hours, and I was dripping sweat by the end, but it made a big difference. Before I stretched them, they were uneven by as much as 1.5 inches in some areas, and afterwards they were all the same length. I have some pictures of the before and after that I will try to find later today.
The way I did it was to put a metal bar through the end of the lines where it attaches to the canopy and pull on it for a good minute. After a couple of lines were stretched, I realized I could lean my entire body back to stretch them without fear of the lines breaking.
If you are uncomfortable doing it, talk to your rigger, but if you are up to the task it can really make a difference for the last hundred or so on the lineset.
Edit: Attached the picture. I can't remember which lines these were and unfortunately I don't have an after shot (I wanted to make the first load) but you can see how out of trim they are. Honestly, if you have the money on hand it will probably be easier to get it relined, and if you are looking for improved flight characteristics you might not find much by doing this. But, if you are feeling ambitious and are comfortable with the idea, then give it a try.
(This post was edited by adumbelk on Dec 28, 2012, 11:01 AM)