Totally depends on what I want to achieve. I have quad tracked with exactly the same riff just with a different sound and guitar. And I used quads only for the single note bass tones of the riff or used a higher version for more sparkle and so one.
I use quad tracking with the exact same riff normally when I want to make everything wider and deeper, it takes everything a bit back and not so in your face like with only two or three tracks. You can edit the s*** out of it to make it ultra tight but normally 2 or 3 tracks does their job too and for tone stacking you can do tone stacking with different reamps.
But sometimes just adding a different guitar brings out the tone that you want. I have a tone for my own music where I sometimes use my PRS and a Tele with the same riff. I try to play them as close as possible and than edit them a bit. That has such a great power with the additional twang and the lovely bass attack of the Tele together with the dark and warm sound of the PRS. They are blended together nearly at the same level but from passage to passage in the riff one of them is a bit more dominant.
If I combine different styles of the riff I normally have a main tone and the other ones are a bit back. So as an example if I quad with a single note bass line I want this to support the riff and not to be totally in front of the main riff. But sometimes it's nice to automate this for dramatic effects.
Awesome! Thanks for the tips.