Well, that is a VERY interesting and, I might add, a very pertainent question. Personally, I think after the snow gets to the skid plate it is time to get out the sled! The "plowing" effect really becomes over whelming beyond that and "other issues" like snow packed frozen carbs become previlent. Before that point, say between 3 and 9 inches is very very special!! If you are "screwed" decently it is nothing short of AMAZING how magic the traction is! I have a REAL close "winter riding" buddie that in the recent years I woudl do ALOT of night riding with (I am talking 2 am @ -5 degrees at Tin Cup :) ) and we purposely would wait for this "perfect" condition!! We even started a ritual of riding "headlightless) in the dark, in the snow just for kicks - WHAT A BLAST!!! I have found that if you are riding with a group of riders in fairly deep snow (above 6 inches) the guy in the front has a major advantage. Even with screws, the bikes "tendency" to follow the previous riders lines can and will make conditions almost unbearable, for say, the guys beyond 4th position...
Another tidbit of info. If you decide to try the "winter thrill", be VERY careful of pavement. Ice is great, trails are AWESOME, even those nasty ol diagonal trail branches are not much of a match for a properly studded set of tires but PAVEMENT of any sort can = FACE PLANT easily!!
Also, another favored trick is NEW TIRES with NEW SCREWS! The more aggressive the tread the better (for woods) as far as DEEP snow traction goes... I have had VERY good luck with Dunlop 752's for deep snow application.
Another word of caution, some of our Michigan ORV trails are also snowmobile trails. Once a snowmobile has ridden the system and their track is "recoverd" the trail speed for bikes goes way down!! Try it sometime, you will see what I mean.
Gusee what. After a winter of riding, I HATE to take my screws out!! THe traction is THAT shocking!
Woodsy