Apparently reiterating what a lot of the replies seem to say, I consider giggling a girly, childlike thing. None of my most mature characters or really masculine characters would ever giggle, and even most of my female characters won't giggle unless they're feeling particularly giddy or silly. My characters who giggle are either very young or are feeling embarrassed or goofy, or they really are kind of girly (including my male characters--I have a certain male character in mind who would not be averse to giggling).
When I think of "chuckling," I tend to think of older male characters, and for that reason most of my younger and female characters don't chuckle. They'd probably "laugh softly" instead.
But that's just me. Consensus seems to be that giggling is kind of immature, though.