Since no one seems to have mentioned it (or my spam filter blocked it),
the ARIN policy recommends both /56 and /64 allocations. From the ARIN
Number Policy Manual:
126.96.36.199. Assignment address space size
End-users are assigned an end site assignment from their LIR or ISP. The
exact size of the assignment is a local decision for the LIR or ISP to
make, using a minimum value of a /64 (when only one subnet is
anticipated for the end site) up to the normal maximum of /48, except in
cases of extra large end sites where a larger assignment can be
The following guidelines may be useful (but they are only guidelines):
* /64 when it is known that one and only one subnet is needed
* /56 for small sites, those expected to need only a few subnets
over the next 5 years.
* /48 for larger sites
For end sites to whom reverse DNS will be delegated, the LIR/ISP should
consider making an assignment on a nibble (4-bit) boundary to simplify
reverse lookup delegation.
RIRs/NIRs are not concerned about which address size an LIR/ISP actually
assigns. Accordingly, RIRs/NIRs will not request the detailed
information on IPv6 user networks as they did in IPv4, except for the
cases described in Section 6.4.4 and for the purposes of measuring
utilization as defined in this document.