I tune my synthetic rig when it's cold out, then in the summer it gets a tiny bit tighter. As has been mentioned, it may be the aluminum
mast shrinking with cold.
Either way, unless you're cruising in places with huge-HUGE-temperature fluctuations, it's hardly the end of the world to give a quick tune when the weather
gets cold, if you're hardy enough to sail in sub-freezing temps.
Which I'm not.