yep, i've done this more than a few times. i think you are right, those of us who have done these sorts of races appreciate how much the cheering can help you. case in point, after my sister and i finished the grueling 40 mile livestrong ride (grueling for us anyway), a few weeks later our niece was running in the philly distance run and we were there to cheer her on and we stayed and cheered on darn near everyone that finished. we were planted, on our bikes of course, at mile 12 and cheering everyone with the - one more mile to go, piece of cake, all down hill from here - comments. some guy coaching a team in training group came up to us and thanked us. we both said, we know how it feels. we both also, more than a few lbs ago, ran a lot of 10Ks.
anywho - if a race is run properly (which lately i have to wonder about some), there should be plenty of folks there to man a water station that your ability to mess up anyone's time is negligible. you just stand there and hold out a cup of water along with a slew of other people and the runners just run by and grab until they get one.some of the more serious runners will run by the station anyway and have water of their own, if they are really worried about their times.
then in addition to the water, you cheer and yell for everyone. i really enjoy volunteering at them (plus i usually got really nice tshirts too!).
go for it, you won't do any damage and as someone that has run them before, you'll do a lot of good for the folks running, especially the ones struggling who need the cheering on.