Why hand turn the prop?
(1) So it's not horizontal then the birds won't sh1t all over it.
(2) I learned at a school that used the prop position to indicate fuel status, vertical meant needing fuel, horizontal meant full to the brim.
(3) I've heard some poeple like to leave the prop horizontal to lessen the chances of someone taxiing past and hitting it.
Note I didn't say it was necessarily a good idea though.
I too have heard the warnings about rotating backwards and vacuum pumps but have no idea if it's really true. I've heard going backwards reduces the chances of an actual firing due to the timing. Don't know if this is true either.
Perhaps (as SSD suggests) rotating dates back to the days of the big radials, the idea being to rid the lower jugs of oil and thus prevent hydrolocking. I've seen films of three people turning the blades of a B24. It looked a little like a Maypole dance.
However, I have read that if you do the mathematics, you can bend the rods just by turning the blades by hand, due to the tremendous mechanical advantage produced, so it really helps to have those lower plugs removed..
Anyway, fooling with the prop cant' be THAT bad.
This is from the Cessna web site]