Ste,
my personal view is there's no point, unless you are constantly moving your decks for gigs etc.
Intenal grounding or external grounding - no difference - the deck is grounded to the local earth.
If you're gigging with them, them you need to keep connecting to other equipment, such as mixers etc and a risk the wires will be broken through use and abuse.
home setup - once connected - its fine
but then again - I'm useless - so what do i know