% Expect to see routers being optimized that will only route
% the upper 64bits of the address, so you might not want to do
% anything smaller than that.
This, if it happens, will be exactly opposed to
the IPv6 design goal, which was to discourage/prohibit
hardware/software designers from making presumptions or
assumptions about the size of prefixes and HARDCODING them
into products.
--bill
Opinions expressed may not even be mine by the time you read them, and
certainly don't reflect those of any other entity (legal or otherwise).