it depends on the size of the screen compared to the plant really. i tend to flip before the screen is full and let the stretch finish filling the screen as well as the buds get a bit of height off the screen as they finish stretching - but then i use big pots and small screens in a modular scrog.
if it doubles then you are going t get a lot of height off the screen if you fill the screen in veg and then let it grow up during stretch. you dont want more height off the screen than the useful penetration depth of whatever light you have.
VG