Some good points raised so far.
Certainly (for the reasons mentioned) using a firm surface such as asphalt or concrete is advisable but grass tends to be kinder on the airframe - less vertical 'shock' (through the absorption of some of the energy by the ground on initial touchdown) and sometimes, depending on the surface, less 'juddering' to the skids during run-on.
Checking the touchdown zone is truly an important consideration and can save much heartache later on!
Its been posted before but its such a good example of a 'classic' 206 auto that its worth repeating:
Back in the day some insurers would get pretty specific about what could and what could not be done when it came to autos. More than one 206 I flew had approval for touchdowns .. with the provision that this be carried-out only when the craft was wearing shorts (ie. fitted with low skids). Certainly the lower centre of gravity with short skids helps.
Would be interested to read a summary of Shawn's 'Pass on the Grass' booklet if someone would care to post the highlights.
Regarding touchdown autos in general; I always found they inspired confidence, especially when taught by a capable instructor. I do believe that getting that 'last bit' right is important!
Bell Academy did all their full-on auto to pavement, and there is more noise (and sparks at night) ..
Have some interesting memories from the US which involved spraying a stream of sparks from a set of 206 skids during night t/r failure training and which involved putting the bird on the taxiway at around 30kts or so but that, as they say, is another story!