Ok here's the deal.
It's dependent on both the media AND the burner.
I own a Liteon 851S->832S. I burn everything at 4x onto 16x media because that's what my burner is happy doing. Actually now that it is 16x media I probably COULD burn at 8x, I haven't tested in a while.
But I do it this way because that's the media I have and the way that works. I used to use some media that I had to burn at 8x or else it wouldn't come out properly.
And yes, modern burners with modern media burn like ASS at 2x, so much so that many new burners don't let you select anything under 4x with 16x media.