I've gotten 3 12 month 0% BT offers in the mail the past week. Discover, BoA, and AMEX.
As said, don't kill the cards if they are your older tradelines... that would do more harm than good to your credit score. You can keep it open but not use it, the bank won't make any money off of you just the same as canceling the cards. Unless of course if there is an annual fee.
For a good credit score it's best to use the card at least once every few months and to keep the card issuer from dropping your account. There is no need to carry a balance as that information isn't contained in the reports (only the current balance is reported.) If anything, carrying a balance will hurt as your revolving credit utilization could report higher, and high utilization is one of the worst hits.