NSEU
I"m afraide to mistake, but yesterday I try to check on the EICAS APU Maintenance Page (CMC) how long it takes the door to show OPEN (13deg) when going from OFF to ON? Nothing indication, absolutely! No N1, Door open, EGT... It means: start sequence does not start? I wait 30 sec then set to START. After that APU start up has been successfully performed. In this case NO reason to wait some second in ON position before start up APU.
anson harris
You are right, there are not papers where do Boeing recommend that, only "a little bird told".