I think users should use either tag whenever they feel it's necessary to speak to one specific gender, otherwise keep things gender-neutral. As a sidenote, I prefer #boyski to #byyski, and #girlski to #grrlski for that matter.
I don't see myself ever using either tag, but far be it for me to care one way or the other about how people use tags. I think it's always cool to see things like tags being used in ways I'd not considered. Go to town byy's and grrl's :)