Hmmm...as someone who mostly posts in one journal (fic journal) and then links from communities...I guess I haven't given it much thought. If they want to comment in the comm entry, it's cool. I get notifications and can respond there. If they post in my fic entry on my fic journal, that's cool, too.
I guess I just post to one place when I can so that if I have to make any changes to the fic itself (spotting a misspelling or a missing period/comma/quote), it makes it easier to do it in only one entry instead of trying to go around to the different comms and make the changes.