TV's Frink said:

I hate bees, but they started it, what with their flying around and their buzzing and their stingers and their hey hey hey...

I suppose they can be a little intimidating. You're generally ok if you see a lone forager. When she's out collecting pollen, nectar, or water, she's not really in attack mode. She has much more important things to do.

Now, as Ziggy says, if you provoke them by getting too close to the hive or swatting at them, the guard bees will let you know. You'll usually get a warning bump or two. If you don't back off, she'll make you back off.

Those guards are at the hive entrance, not out foraging. They do make periodic flights around the hive, so they're usually withing 10 feet of it. That's really the only time you're pushing your luck. If you do happen to swat one and kill it, it's time to leave. That releases a pheromone telling the others that danger is present. Rest assured, on that second wave the girls won't be giving out any warnings. ;-)