I stretch the hell outa mine. After I first put em on, I tune up to standard tuning then grab each string individually over the bridge with my finger & thumb and move it up and down until it goes outa tune. Then I tune back up and repeat the process until it no longer goes outa tune.
Basically, stretch it as far as you can without breaking it.

I use custom lights (9,11,16,26,36,46) & never have any problem.
Like I said,you always have to do a slight tuneup just before playing because of temp and humidity changes. You might have to retune after playing awhile too because your hands warm up the guitar and strings. It should only go slightly out of tune tho.

I think I did a video tutorial awhile back covering restringing and stretching if you have full access here.