Warehouse 13 isn't fluff; it's a good, solid adventure show. The term fluff implies useless filler-- that's applicable to infomercials and reality shows.
As for when is the right time to cancel a show-- there is none. There will always be people who want to keep watching and people who don't (although it's a lot easier to stop watching when a show is still on the air than it is to keep watching when it's off).
At the convention, Rene said he was asked to do an appearance on one of the last six. He said he was going to try to do it, but his schedule was tight.
That would be nice. I'd like to see all the major recurring characters show up at the end if possible.