If it's only a hairline crack probably not a problem. Normal for the unyielding metal of the keel to work against the more flexible fiberglass. It's especially evident if someone tried to glass over the join or used a not flexible sealing compound. Fill with Lifecaulk or 4200, smooth out and paint over.
I like this answer. Yes it's just a hairline crack. The boat gets a coat of ablative paint at the end of every summer, I'll see how much paint I have to remove to get to the glass to fill it in. I just really wanted to know if periodic torquing of the bolts is a routine maintenance task that I should have been doing all along which may have prevented this. It feels more like normal wear and tear, but it's a bit nerve wracking to see cracks on the bottom.
Thanks for everybody's replies.