I trim both when wet and dry, I've read that it helps 'break down carbs and retain more sugars' to trim after most of it is dry, but I don't know about that. I do it because I like cutting off all the green shit that has no trichs, so that when I make hash using the trim, it's mostly frost and not green matter.
I usually like to take off all the big shit first though.
And yeah, I haven't flushed in well over a year, I think flushing for the last couple weeks or whatever people do affects yield and such, plus I'd say the only time to flush is to leech excess salts.