I worry less about antibiotics and more about the poo that gets mixed in with "waste-cow" ground meat. E-coli for one example is fecally carried, and the potential for poo-mix is HUGE when the folks at the factory are salvaging the last little bits that go in to the burger grinder.
Can't avoid the potential if you buy factory-grind, but you can protect yourself from illness by killing off the little buggers:
Cook pre-ground burger to 170, folks- that'll kill just about anything. It won't improve the taste any, though.