What Gazza means is, if it's climat control (where you can set a specific temperature) then just leave it on. It both heats and cools automatically.
Manual air con is basically a chiller, and as stated above should be used at least once a month. Aside from bugs there can be corrosion due to condensation if you don't - air con dries the air too.
As to not letting it blow on your screen - well the fact is that when you turn the air distribution to demist the air con automatically turns on - so you can't always avoid it. The steaming up phenomemon can occur when turning on air con on warm humid days - there is initial misting before it clears. I've never had this on the move personally, always when starting a journey.
2007 C-Max 2.0l Zetec auto in Panther Black