Only stretch your lead came just enough to make it straight. If it’s already straight there’s no need to stretch it.
Stretching lead came makes it temporarily more rigid – but as the lead came relaxes and goes back to its original size, this extra rigidity is lost. Furthermore, weak spots will be formed that can be the origin of small cracks and corrosion that may cause early failure.
As one expert in the care and restoration of stained glass, says: “The only reason, albeit a good one, to stretch lead cames is to make them straight. As soon as this is achieved the stretching process should stop.”