Fast, Fast, Fast.......Dont think of rebound as something that is slowing your wheels down, think of it as something that is holding your wheel to the ground, Set it fast, but not so fast that your getting bucked.
With a sufficiently fast rebound you make sure both wheels keep sticking to the ground.
I had a manitou sherman that was very slow packed up on multiple bumps and gave a harsh ride. That fork would have been slow for everybody.
But I think rebound isn't something you can discuss without thinking about the LSC. If you enter a berm with not much lsc then your weight will shift a lot giving a unstable ride
if the rebound would be fast to you will pogo trough the corner.
If the rebound is slower then you stabilize the bike on the rebound stroke.
Anyway I think you should use lsc to stabilize the ride and use rebound to keep the wheel tracking the ground.
For me, it depends on riding and track conditions.
There are a couple of tracks I race at that pretty much groomed all the time, so they have a super smooth and hard packed surface. In these conditions I run slow rebound rear and slower front. This helps me keep more momentum through turns by keeping weight that transferred form braking, low through the entry, apex, and exit of the turn.
On normal riding and rough conditions, I usually run rebound fast front and rear.
Go feel Straight's or Kovariks bike, then tell me to slow it down. Only time I slow my reblound is for freeriding and dropping, or big ass gaps. For racing its all about keeping your tire on the ground
But in reality, ride whats best for you, for me its all about what terrain I am in, And what I am planning on doing with that terrain
I would like my rebound to have both high and low speed circuts so that with high speed impacts it's fast and with low speed impacts it's slow.
Baring a perfect world, a little on the faster side because when I go faster and get more high-speed impacts, that's where I want my suspension to work optimally. Packing up at speed decreases traction.
Front fast, rear faster than average but not as fast as the front. I run enough compression (LSC especially) that I don't have stability problems with fast rebound, and the bike certainly jumps/hops a lot better than with fast compression and slow rebound.
I think as a rule of thumb, if you're going to consider nothing else - the front should err on the fast side and the rear should err on the slow side. You can run your rear end fast provided other factors make it suitable (eg plenty of compression, faster front end, not oversprung relative to front end) but for safety's sake the rear end should always be at least a tad slower than the front.