vibration in steering

my girlfriends 98 ford escort has a constant vibration. steering is tight, tire pressures correct, no growling or clunks. I haven't checked ball joints or tie rods due lack of time and mostly laziness. I am leaning towards an out of balance or separated tire. Sound correct?
Tires are old and and worn almost down to nothing..we are getting new tires for winter. Just curious if the tires are most likely the cause of my vibration.