that fuel "sitting" on the valves actually helps to vaporize the fuel before entering the cylinder. you have to remember that the intake valve is comparatively hot, and this helps the fuel to vaporize further when fired on it.

there's of course more physics to it than that, just saying it isn't necessarily a bad thing...

 

well, on a V8 motor idleing at 750rpms, if the injector sprays just as the valve closes there is just over a tenth of a second before the vavle opens up again. probably hardly enough time to precipitate and puddle on the vavle and then heat up and vaporize. of course, it would be closer to .22 seconds on a 4 cylinder and somewhere in the middle for a 6 cylinder. and cam durations have an effect too when talking about how long the vavle is closed vs open. my math used an adv. cam duration of 260 degrees which means the vavle is closed for 500 degrees.