Just back from a rather excellent lunchtime seminar at the RCAHMS on cup and ring markinga (part of Scottish Archaeology Month).
Three of the major findings from the recent survey up by Loch Tay;
-most of the sites were on well drained soil and most of them were on boundaries between well drained and poorly drained soils.
-sites were clustered around SW flowing watercourses.-
-the most elaborately carved sites *were on the highest ground*.

I put it to the presenter re the poorly drained soil question- could it have been a purely 'comfort' thang? In other words- when walking up the hills, you would naturally choose the less wet route. If you were going to carve a rock over any period of time would you rather walk there on dry grassy hillside or wade ankle (and knee!) deep through bog? (I'm sure you'll appreciate that FW!) Things are bad enough in goretex lined boots....
I never got a chance to add my tuppence worth re the more complex and higher sites, but I personally think it might go back to the viewfnider/map theory. The chairman of the seminar also suggested that some carving may act as route markers when lost in the hills.
Finally, it was concluded that these stones must have numbered thousands- the landscape would have been full of them. What we are looking back on is essentially a 'lost world'- it is doubtful we will ever know their meaning, but imagine what a fantastical landscape they must have been in......