I stretch new strings a lot, by pulling on them several times along their length, when I put them on. I also press down just before the bridge. Maybe it's only psychological on my part, but I hardly have to re-tune after that. I change strings every 4-6 weeks, depending on how busy I am and I generally put on the new strings directly before a gig. It would be very unwise of me not to stretch them before going under stage-lighting and those kinds of temperetaure changes.