RE: DS821 Servos
Yes, they are fine to use on 6V. 6V will shorten the life span slightly as with any servo, however this is not significant.
Specs
Type: Digital Hi-Torque
Torque: 72 oz/in @ 4.8v, 88 oz/in @6v
Speed: .19 sec/60 deg @ 4.8v, .15 sec/60 deg @ 6v
Dimensions (WxLxH): 0.74"x1.50"x1.47"
Weight: 1.5 oz
Bearing: Single Ball Bearing
Motor Type: 3-pole ferrite
Gears: Composite
Application: Standard size-digital servo upgrade for sport pilots