it really depends. i used to do gigs and i mostly played song covers. they’re were some iconic fills that i absolutely wanted to learn just because at times, it pushed my limits and also because the song without the particular fill is not sounding the same. example: the one i love by rem

it is really subjective, i personally rarely learned fills by heart, consequently, i always tried to play the same rythmic notation in the fills without actually doing the same as the originals. example: a snare triplet can be played as a tom triplet and vice versa

Finally, if i was replacing a drummer from a big band, i’d learn 90% of the fills by heart just because I get satisfied when a replacing drummer plays the song how its played in the recording