Though it is verging into another debate altogether I do think you need to clarify what you mean by 'filler' mate? I don't think any DJ neccessarily goes out to play a boring track simply because it may mix well together with another track.
Different genres dictate different needs as well, if I play an opening progressive set & start whacking out big tune after big tune then it basically f***s up the flow of the rest of the night & thus lessens the impact my set would have, whereas I would never claim to be playing 'filler' material I would definately seek to consciously avoid playing really big tunes one after another.
Getting a set to flow properly does dictate that you need to play some tracks which maybe don't contain big riffs or big breakdowns & simply keep a nice rhythm going & keep people moving.
If I've misunderstood the term 'filler' then by all means discard what I've just said