You'd be doing your guests a disservice if you didn't include a quick note on your invitations about the dress code. (No one wants to be the one to show up utterly over or underdressed!)  Your dress code info goes best on the lower right corner or at the bottom of the invitation with a small asterisk. A simple "casual attire suggested" or "cocktail attire" should do the trick. Or even something like "casual, comfortable attire encouraged" would work if you're throwing a backyard barbeque soiree.  If your wedding invitation is too jam-packed with other important info, you can always list the dress code on your wedding website (Minted makes that nice and easy!).