Welcome to Librarium Online!
I've been playing Necrons for a few months now, and I am no expert but I like to give advice out when I come across something in a game which totally hindered my battle.
And this was the direction of my Monolith's portal --
To quickly summarized the situation: it was against Tyranids and the player was able to immobilize my Monolith on the second turn; though he never destroyed it in the end. Even so, my portal (front of the Monolith) was smack up against a building and you all know how the Tyranids love HtH; so I was unable to portal my warriors out of combat and eventually succumbed to the swarm.
Since then, I am VERY conscious of the direction my portal is facing, because I think to myself after I move it - "If it becomes immobilized, can the Monolith still be effective and teleport troops out of HtH."
I hope this helps some of you (and doesn't offend others) as it is just something to ask yourself after you move your monolith.
Necron Govna' on Dracon_Prime