In my mind, it depends on the rhythm involved. A tremolo should sound sort of halfway between a "drum roll" and a rhythmically-repeated figure. What that figure is varies from piece to piece.
Here's a fairly slow tremolo from the "Funeral March" in Beethoven's Sonata Op. 26:
As you can see, the notes are 32nd notes in 4/4 time, and are played quite strictly as such.
Now, here's a passage from Beethoven's "Tempest" Sonata, Op. 31 No. 2, with Claudio Arrau:
Now, these aren't octaves of course, but the principle is the same. As you can see here, the notes are 8th-note triplets, in a faster 4/4 time, and have to be played quite rapidly. You can see that Arrau sort of attempts to play them as triplets, but that is quite difficult to do. You have to accent the bottom note of the tremolo in one triplet and the top note in the next. You'll notice that he succeeds in doing that in some places (in particular the ones with two notes on the bottom and one on the top, instead of the other way around), and in others the bottom note is always louder than the top.
Here's another version, with Maria Joao Pires. I think she manages to get that rhythm a little more clearly:
So, my take is that it's important to preserve the meter of the piece when you're doing tremolos. It also isn't one of the most important details, typically (in this excerpt, you're paying a lot more attention to the notes in the bass than the tremolo, for example), but there are no details that are beneath notice.