I'm of two opinions here - one, if it is someone's housepet, taking it to a shelter will make it harder for the person to find it, if they expect her to come home in a few hours. And what if they are the type of person who will not pay a shelter to get their cat back?

But on the other hand, people who let their cats roam in the city with out collars don't deserve a pet (in my opinion)..

But then again, maybe she is not normally an outdoor cat, and happened to escape accidentally, and someone is frantically looking for her? I hope they think to call the shelter.

Make sure the shelter scans her for a microchip!