They have only been on about an hour and so far looking damn good.
Very nice!
3456A also tend to be very stable calibration wise.
Welcome to your next little step t'wards voltnuttery.
A good reason to extend your collection of reference voltage sources and precision (reference) resistors..
But..
pros:
No backup battery that could leak.
cons:
3456As reset at power up.
But who would power a 3456A down anyway?