Technically, Marriott could certainly ask you for corp ID to use the rate. In practice, they typically don't.

It's been discussed a lot here. If you work for Company A but are traveling to do business with Company B, are you entitled to use Company B's hotel rates? I think the general take is that you are unless Company B or the hotel itself specifically prohibit it. Of course, with any client I've had in the past the opposite has happened - they've been eager for me to use their codes and preferred hotels. (Some of my clients have gone so far as to require it.) Obviously it's good for the company's budget, and it's good for the hotel's overall relationship with the company.

I figure if a Baltimore-based company is negotiating rates with Baltimore hotels, it's so people coming to visit them on business can use the rates. "Doing business with" is kind of loose, but if you're a vendor, partner, supplier, consultant, etc. it seems like you should be good.

Government rates sometimes have wierd rules. That's another topic altogether. Showing up in DC to do a pitch for a government contract probably doesn't entitle you to use government rates.