I've had just a drop of water change the appearance of the anti-glare coating on my T430's matte 1500x900 screen. Once applied
very gently with a soaked soft paper napkin, to remove a new speck on the go because it was irritating me and also before it dried and "set" on the screen. Once when a drop fell from my washed and rinsed hands, while reaching for a towel in cramped quarters on vacation, and rolled partway down the screen before I got to it.
The marks don't show much while I'm using the machine and looking directly at the screen, unless something particular is displayed at those spots, although I can see them if I look for them. Quite evident when the screen is off, particularly if there is any reflectivity (hazy reflectivity, due to the matte screen).
Because of this, I've become very hesitant to clean it. I'm pretty clean and careful, so otherwise the screen doesn't really need it. I very gently wipe away dust with a disposable "Swiffer" type cloth -- although I've started worrying about the long term effects of minor abrasiveness.
Anyway, I would very much like to clean it and all the more so, my W520's very nice full HD screen. The latter got some spatter while I had an acute medical condition of some weeks.
But, with simple water apparently altering the matte coating, I'm very hesitant.
As for blowing compressed air in the vent holes. I would be worried about blowing dust and debris to a different location where it does more damage. And, with desktop machines, I remember the advice making sense to carefully freeze the motion of fans with a pencil or similar, to keep the compressed air from torquing or over-revving the fan and excessively stressing its bearings. I would assume that could apply to laptop fans, as well.