Ideally, replacing both at the same time is preferred. Like changing tires... you wouldn't replace 1 front tire and leave the other one with only 30% tread?
Really, you should be ok; I mean the only negative affect I could foresee is alignment.
Worse case scenario one side tire wares a tad faster then the other, and that’s at a long shot! (ie. if the bushings were shot or something)
So honestly, fuck-it he's just trying get more $ out of you.
