Isn’t IPA also hygroscopic, just like the ethanol content of car fuel? Fresh out of the tightly closed bottle, it probably won’t make a difference if you use it up right away and the amount apperars to be small – but if you leave the tank half full in a humid environment after your flight? Probably still low, but more than without.
I was under the assumption you would want to avoid water in the fuel under all circumstances.
Yes; I add 1 litre IPA to my two tanks (86USG total) where water-contaminated fuel is suspected (some places S of the Alps).
Best thing by far is keep the fuel cap seals in perfect condition. Very few people do this. I once drained about a litre out of a PA38 
