Filtering multicast and/or 6to4?

[Quartz]

So I've tried asking this on -net a couple times, but they're completely 
ignoring me for whatever reason, so I guess I'll try here instead.

After all these years, I'm still not 100% sure I understand multicast 
and 6to4. I'm trying to figure out when/why/if I should be filtering 
stuff, and in which direction(s). Is there anyone here who's familiar 
with this who might be able to help?