"All Apologies" mixes basic single-note patterns with one-finger power chords. This is a nice easy song, which even the most beginner guitarists can make sound good quickly.
When playing the single note theme to the song, you can use all down-strokes, hitting the low E string when specified to fill out the bottom end. You can get away with playing the open sixth string more than is specified in the tab - it will sound good wherever you play it - but be careful not to strike the open sixth string too hard. The "main" notes of the theme are played on the fifth and fourth strings - make sure those are the notes which ring out most clearly.
In true Nirvana style, the song is tuned down a half-step, but Kurt has additionally tuned down his low E string another full step, giving us the opportunity to use play this low E string throughout the single note theme. Because of the dropped sixth string, the final tuning for "All Apologies" is Db Ab Db Gb Bb Eb.