I think that for slow songs, say 125 bpm or something, i usually do (just an example) a fill of 2xsnare (with one hit on the cymbal as well),and then 2 hits on each tom. But in fast songs, i think you should just pop in as many hits as possible...without overdoing it ofcourse.

an exception would be a slow but aggressive song (like most of Six Feet Under's songs) would also require a lot of drum-pounding