top tube dent.
 

if there is a little dent on the tt of a steel frame, can that be hazardous?

Little? Steel? Generally, no (I think). Unless it's really thin.

Only to its resale value.

My Mercier came with a TT dent thnks to UPS and I
tweaked the frame somemore RotoF***ing the cog.
No problems(yet :eek: ) and it gets ridden hard.

If it's freaky-thin, you can pop it back out with compressed air.

I have two dents in the tt of my steel

It depends-- if it just looks cosmetic it probably is. If it affects the frame alignment because it's seriously whacked then it probably needs a new top tube.

Pretty much if you see any creasing in the metal, then its potentially serious.

But generally, any small smoothish dents ain't going to be a problem. If you're worried get someone who knows what they're talking about to check it out.

Failing that, post a picture here and we'll chime in.