WEll you are not going to guarantee getting upstairs sills spot on and gleaming it's true, but I'll bet most of us, prior to going trad, only gave them a quick wipe with a wet scrim or quickly nipped over them with the squeegee to blade the excess water off.
I for one never (or rarely I should say) rubbed them thoroughly with a wet scrim and then dried them with the dry scrim.
Spiders and other creepy crawlies are not a problem though, when you do the bottom edge of the window frame you scrub them out no problem, beseides, for the most part they are always in the top corners or tucked away on the top edges.
My own technique is generally to give the upstairs sill a quick wipe over with the brush as I complete the window.
I have a couple of customers who are very fussy over their sills, and I manage to keep them happy, so it really isn't much of a problem.
As for pure water being more corrosive...technically it may well be, but by the time it hits any bare wood or goes through splits and cracks in the paintwork it is almost instantly contaminated.
Condensation will settle on paintwork all the time, and THAT is purer than rain water as it hasn't fallen through the atmosphere...at least I reckon it is purer! Although I suppose you could argue that as it is part of the atmosphere then any contamination is already present
Ian