This was actually queried at a fairly high level (I think it was in BA) and the word was there is no need to wait 30 seconds on the ground. I suspect, reivilo, that we are looking at company specific OMs , as my last company SPECIFICALLY detailed that you do not need to wait. We follow the book (subject to our discretion, of course!)
Pmonkey - correct, but the words were "discharging a fire bottle on the ground, seeing as it shuts off the fuel to the engine" - when the fuel is already off.
Incidentally, the logic in discharging both bottles when there is evidence of a fire in the vicinity is to avoid the bottles exploding in any fire and thus if you abort for some non-fire related event you need to consider before firing the bottles. The same reason goes for discharging the APU bottle.