Should an OFFLINE database be SET EMERGENCY before dropping?
Note: make sure you have a final full backup of the database before you drop it, just incase if you need to restore.

Normally, EMERGENCY mode is used for getting your database out of corruption.

You are right, that EMERGENCY mode will allow only sysadmin members and the db will be read-only - nothing gets written to T-LOG.

I would suggest that get the database to RESTRICTED USER and then drop it.

e.g. ALTER DATABASE your_db_name SET RESTRICTED_USER or READ_ONLY

Note: This is just my preference which distinguish between EMERGENCY mode --> use to take DB out of corruption vs RESTRICTED_USER or READ_ONLY --> give it chance to breathe before you drop !