What should I do about my check-engine light?

A few days ago the “Check Engine” light came on in my 2002 Toyota Sienna minivan. I have not noticed anything out of the ordinary with the van either before or since the light came on. I called a local Firestone and they said they would run a $90 “engine diagnostic” to determine the problem. From what I understand, the check-engine light relates to emissions.

My question is this: Should I pay Firestone $90 to check this out? Also, what are the implications of not having it diagnosed/fixed? I don’t want to increase emissions for environmental reasons but I am also concerned about safety as well as possibly causing more damage to the vehicle.

Thanks in advance for your help!