I have read that same bit of advice a lot of places -- that a microcontroller should only be a current sink, and not a current source. Thinking about it, though, I don't really see why. The microcontroller is passing the same amount of current. The internal dissipation is the same, since the current is flowing through the same components... It's just as easy to put a pulldown resistor on an output pin as it is to put a pullup resistor there.
I don't really see why you can't have electricity flow out of the pins.
And is it electron flow or conventional current we're talking about here?