And if you're going to capo, tune once the capo's on: unless you've got a very perfect guitar, putting the capo on may throw your tuning (perfect on open strings) OUT.
Bruce B: yes, very good in theory: I thought I had a pretty good ear until I bought a tuner; As RB says, a tuner is very helpful in noisy environments. Mine did stop working, but a new battery cured it.