When to use Vacuum?

I'm putting some large read-only data sets together and occasionally I
change my mind about something, drop a table or two and replace them
with something different. Do I need to use VACUUM when I drop or
re-arrange tables? Or does PG release disk space when you drop tables?
And finally, if the tables are strictly read-only, then do I need to
use VACUUM at all on a regular basis? (Like, do selections take up disk
space that needs to be cleaned up on a regular basis? It's my
understanding that VACUUM only clears or marks for re-use deleted
records, but the docs say nothing about deleted tables and I don't want
to get into an intolerable situation someday (like disk full.)

TIA,
- Bill Thoen