Absolutely worth paying a little extra money for a 0/0 seed tag (0% weed, 0% other crop). What's worse than weeds? Knowing you actually paid for and planted them. 0/0 seed options available now at Heritage Landscape Supply branches.
Pay attention to your seed labels! Note that “other crop seed” would include things like ryegrass in a tall fescue seed. 1% is enough to leave one rye grass plant every foot in every direction.