I used to be responsible for teaching immigration regulations in the travel industry and would strongly encourage using separate, appropriate lines, since that is what is officially called for. (There are exceptions, i.e. long lines, that may allow for agents flexing the rules).

Way to many examples to cite, but others are correct in that a non-USC traveling w/ his US gf could raise a red flag to INS and make them wonder if your bf would ever actually be leaving the US... Don't lie, just follow the rules and keep it simple. If you're still unconviced, I'd be happy to recant the crushing results of those who tried to skirt the system. INS agents are usually polite pleasant, but there are just as many who are spectacularly humorless about the process and unforgiving to those who try to screw with it...