Hi Steve,
I have three skeps and never cleaned (cant spell sterilised) any of them.
When you consider how long the swarm are going to be in a skep then its really minimal, as they need to be placed in a Nuc or Hive asap.
So lets look at what disease a swarm might have,
AFB / EFB No.
Noseam - possible so check it out.
Acarine - again possible so check it out.
Carrying Varroa - probable, so dust and count the mite drop and treat accordingly.
Viruses - we just don't know.
I would not bee too concerned with sterilising the skep, but I suppose AA might help.
Regards;