I agree with snojock. There is no magical answer to the edge bevelling question. There are a lot of factors that play into what kind of bevel a tuner will achieve when tuning a pair of skis, and there are even more factors that will determine wether the skier will notice the difference or even wether the bevel will make a difference.
When tuning a pair of skis, just like a lot of repair/tuning processes out their, the tools are what make the difference. I firmly believe that I can achieve an accurate, consistent bevel using a good quality file guide for the side edges, and a set of swix base bevel sleves for the base edges. By using any thing else, I think that your beveling is merely an estimation (except maybe a stone grinder).
For example, when you use a Wintersteiger or similar Wet Sander to grind your bases, the machine will effectively bevel your base edge about 1º (beacuse the ski is pushed into the belt, which is supported by a soft rubber roller). However it is nearly impossible to accurately control the amount of pressure being applied by the machine. A stone grinder on the other hand will give you a very accurate 0º base edge bevel, but does nothing of course for the side edge.
Therefore I believe that hand tuning is the only way to achieve accurate bevels. But only an experienced and capable tuner will be able to ensure the accuracy of his / her bevels. Some factors affecting the accuracy of the bevels could include but aren't limited to file flex, collection of filing on the file, a non-flat ski base, gouges or imperfections in the edge (including Tempering), etc.
As far as being able to tell the difference, the skier must be capable of detecting suttle differences in the action / reaction of his/her skis, and in most cases this means that the skier would be more advanced. Also the snow conditions are going to limit a skier's sensitivity to differences in a ski's edge bevel. Really hard snow or ice would be the best conditions for detecting beveling differences.
For an idea on the original post, the point of "de-tuning" a ski is to make turn initiation / completion easier by dulling the edge where it loses contact with the snow due to the curve of the ski. You wouldn't lose any effective edging if you were to detune your skis properly, which I consider using a gummy stone or a diamond stone to smooth down the edge (take some of the sharpness off) where the ski begins to curve up into the shovel at the tip and tail. A lot of beginner tuners will detune too far down the edge, and that's where you start to lose some effective edging. I don't think progressive edge beveling is the right way to go about de-tuning a ski.
Just my $0.02
<FONT COLOR="#800080" SIZE="1">[ January 24, 2002 04:30 AM: Message edited 1 time, by Powderhoundin ]</font>