I recently rewatched/marathoned the series a couple of weeks ago, and there is quite a large collection of filler episodes placed quite a bit in the series. Personally i wouldn't call them filler episodes, because pretty much each of those episodes includes some character growth, and if not that, the plot is sometimes moved forward just a tiny bit. Still, i loved those episodes quite a bit, and find them very enjoyable. It's just more time with this fantastic cast, and that's what i want at the end of the day.