you've always had to choose beforehand if they are coming from the lith or the board edge because if the lith is destoryed before they arrive they cannot come on.
my rule of thumb is ALWAYS roll for the warriors before the lith then you mostly get it to work in your favour. (also the warriors count as disembarking from a stationary veichle so can act normally when they arrive)
Necron Codex says that if you have Necron Warriors in reserve then they MUST enter play through the Monolith if there is one. The 5th ed. FAQ has updated this and it now means you don't have to.