I think it is a combination of Mark and Michelle's answers....If you are running a program and use "declined" as a metric that would be program specific (such as they "declined" an offer specific to that program) then I would say it's worth it.
If you want to indicate if the lead "declined" your company's services or products entirely, then I would do what Mark suggested and indicate that in the Lead Status field or a Life Cycle Status field you have.
tm