The filter is all about odor. If you're not worried about visitors or neighbors smelling the plants, i don't think you need a filter. Unless noise is an issue, but if odor isn't and issue, noise probably isn't either.
My concern with exhasting into the attic is that the humid air introduced into the attic can cause decay to the roof timbers, over time of course.
I don't know if I'm right but I figure if i'm venting out of my roof and not anywhere near the neighbours windows no smells should really come down. hot air rises and all that.
Do not just vent it to the attic if you get winter were you live. Trust me on this one. I had rain in my family room because I did exactly that. My wife is not happy about that one. VV