While I agree with Jpod that it is a bit of an administrative fiction in such a situation, I'd put it a little more strongly than "it couldn't hurt." I would absolutely recommend that there is a deferral election in place before writing a check, etc.
The requirements of 1.401(k)-1(a)(6), which also refer you back to 1.401(k)-1(a)(3), are such that you may have a difficult fight with an IRS auditor if there isn't an actual "election" in place. If there isn't, then of course as Jpod suggests, you would have to make your case, but it is so easy to avoid trouble by putting an election in place prior to any contribution that I can't see a valid reason for not doing so.