Very interesting question.
I started doing all my own routine tuning and repair only last season so I will defer to those with more experience.
This is what Tognar Tools says about shaped and powder ski tips and tails:"Constructing these makes it tougher for manufacturers
to prevent some warpage from creeping in during the curing process. The result is usually slightly concave tips and tails". [Tognar Catalogue at pg. 13]. This matter is not confined to Atomic skis though I recall, as you do, some vigerous discussions here on the subject of whether concavity occurs by design or otherwise.
Tognar recommends removing the concavity with a file or base flatenning device if the concavity is slight or getting a base grind if it is more pronounced. If the tips and bases are very concave Tognar recommends getting the bases flat at least 3/8ths inches in from the edges. [Tognar Catalogue at pg. 13].
If you wish to leave the base "as is", it seems to me that you have four other options as well as running a tape down the center:
1)You could simply ignore the discrepency as most people probably do unless you find that the skis feel hooky.
2) Mark off the concave areas at the edges and use a higher bevel equivelent to compensate for the approximate degree of concavity. The Beast base bevel guides come in .5, .75, 1.00, 1.5 and 2.00. Smooth out the transition area with a diamond stone;or,
3) Eyeball the degree of concavity and mark off and hand file to compensate for the difference and smooth out the transition area with a diamond stone;or,
4)Slightly and lightly detune tips and tails but only if skis feel hooky.
I take skis that show some concavity at the tips and tails to a reutable shop to be ground flat on a stone grinder and have the initial bevels set per the factory specs. Some shops use their own standard machine bevel specs and don't vary them. Since a shops standard specs might not be the same as your skis, choosing a reputable shop can be important.
I then use a Beast bevel guide with a fine diamond stone to lightly polish. I don't touch the base edges after that but only tune with the Beast side edge tuner.
Good luck. [img]smile.gif[/img]