Just to enlight you about the WAAAGH rule of the last Ork Codex:
One use, at turn 2 or after, for one (ork player) turn
To use the Waaagh, the character must have the Waaagh rule and must be the Warlord (so only doable by a Warboss taken as warlord or another character that are warlord with the 1 result on the Warlord traits).
Affect all units with all their models having the 'Ere we go
rule (= reroll one distance die on assault): they can charge after shooting (normal), sprinting (waaagh effect) and maybe coming from reserve (but this, I am not sure)
(It works on most foot units like Boyz, Nobz, ... but not on Gretchins unless the Runtherd is the only one remaining)
So it's not whenever he want and last for his current turn only ( oh and if he didn't declare it at the start of his turn, before moving or reserve, it is invalid).
As it is generated by a model, the model must be on the table. So no warboss coming from reserves and declaring a Waaagh...
No +1 attack from the waaagh (Furious charge maybe ? but IIRC it gives only +1 S)
No Fleet rule too (but 'Ere we go allow to reroll one assault distance die).
See this if you want some details about Orks tactics and rule description: http://1d4chan.org/wiki/Warhammer_40,000/Tactics/Orks%287E%29