When do you tell your spouse?

We live in the town where I grew up. Different neighborhood, but the same actual city. We attend the same parish where I was baptized. I even grocery shop where my mother does, most of the time. Hey, I come from a long line of creatures of habit.
That being the case, it's always a possibility that I may run into someone I used to date. When you consider the kids' activities too, it becomes more likely.
So I ask you, Gentle Reader. When do you tell your spouse you ran into an ex? If it's your date for the Eighth Grade Graduation Dance that you talked to three times in high school after that, and you saw the person pulling into a gas station as you were pulling out, it seems relatively minor. It would involve more explaining than seems worthwhile, almost. On the other hand, if it was a years-long relationship and your kids are in swim class together, it seems to bear mention.
But then there are gray areas--you saw him or her grocery shopping or at the park. Just a weird fluke of timing where it may never happen again. Or you happen to stumble on to their blog and it may be interesting. Or you realize they've stumbled onto yours...
If you tell, when? And when does it cease to be harmless omission and become deception?

Discuss. Or mull it over the weekend. That's why I'm posting this on a Friday. And please, remember to be nice.