If he/she is being a fink to keep someone from being hurt, or breaking something, then i think a thank you is in order.
If they are being a fink, just to be a fink, then no.
I'm with Laurie. It's according to why they are finking. On a whole, no I don't. It gets on my nerves when they start to tattle tale just to see someone else (usually their sibling) get into trouble.