my brother who fitted windscreens said it was largely a myth and, unless it was an absolutely battered or stonechipped / overly repaired / cracked screen previously where moisture could get in, that was likely to never happen
Try telling that to the bloke on the Megane forum, bought his new Mk3 Megane 250, had it three weeks then there was a frost, poured hot water on and it cracked horizontally right across the screen!