I've been running a 9spd drivetrain since 1999ish. I've not had any problems I could attribute to the one extra gear. This summer, I converted it to a 1x9 and run a 32t up front with an 11-32 in the rear. The 9speed cluster gives me a nice wide range of gears.
Oh, and until this summer, the crank was the same crank that shipped with the bike when it was an 8spd. The only things I changed were the shifters, cassette, and chain.
Chris