There is no fan that 'draws air over the packs', what you must mean is the fan that is part of the air cycle machine that draws air through the heat exchangers and that is mounted on the same spool as the compressor and turbine. If there is no bleed air then that air cycle machine won't run, and neither will the fan. Your SOPs should normally dictate what you should be doing with the pack switches on shutdown, but usually they stay on. Back in the day it used to be best practice to keep the APU running for a minute without bleed load (either with packs off or APU bleed off), the NGs do this automatically, so there is no reason to keep them off.