I was wondering if it would do any harm to clean the rear hubs in a parts
As long as you're not planning to use them again, there's no harm in that.
Seriously, the hubs contain two bearings, a bearing spacer, seals, and
grease. Putting them (assembled) in a parts washer would, at the least
dilute the grease with cleaning solution and at worst, remove all the
grease. In any case, you would be left with unusable hubs (no way to get
the cleaning solution out and no way to get new grease in). If your hubs
need rebuilding, they require a special tool to disassemble them and special
tools to reassemble them. It's definitely not a job taken lightly. There
are several sources (TRF, BPNW, Vicky Brit, etc) that sell rebuilt hubs on
an exchange basis.
Hope this helped,