I came into mtb from the road and have always run clipless pedals so I’ve always set them up so my big toe joint is over the pedal axle which in my case means the cleat will be set as far forward as possible.
Now it seems there is a school of thought that for off road the cleat should be set much further back giving a more stable position for descending; but presumably less power on hard climbs.
So interested to hear any comments or experiences before I start changing things around.

At the end of the day SPDs are there to improve the strength of the weakest parts of the legs (the feet) and IMO they should be set up for max pedalling efficiency.

The caveat for MTBing is that dropped heels allow full leg extension while pedalling on a low saddle whilst at the same time allowing you enough movement to keep your CoG over the BB when descending.

Have a read of this http://www.stevehoggbikefitting.com/blog/2011/04/power-to-the-pedal-cleat-position/

Thanks for the link – makes interesting reading.