Nothing in Bebop are what you could call "filler" episodes. Filler episodes are not non-main plot episodes (Bebop might be said to have four main plots anyhow, Spike's, Jet's, Faye's and Ed's). Filler is what happens when a show forgets it has a plot for half a year at a time. You could delete filler from existence and lose nothing about the characters or world because they might as well not matter when the next episode rolls around anyhow.
On the flip side, nothing in Bebop is really filler because it all contributes to the whole that is Cowboy Bebop. You learn something real, something vital about at least one if not more than one of the cast in almost every episode.
Bebop isn't about the plot (the largest plot, Spike's, is just a simple mobster revenge story when you get down to it, nothing spectacular in and of itself), it's about the feeling of it. It's like the music that permeates the series.