Door-to-door salesman should always be treated a bit more cautiously in general. That said, we recently read about a heating bill energy scam where folks were receiving offers from the mail, door-to-door salesmen and unsolicited phone calls. The scammers offer to save you tons of cash on your heating bills.
After you sign-up, you find out your bill is actually higher due to new fees and hidden charges with the new company.

Rule of thumb... ALWAYS check a company's credentials with the Better Business Bureau before signing up. Unless you are consuming huge amounts of energy per month, most of these alternative suppliers are not going to save you money.