If the opportunity presents itself castling is a powerful tactic in the Necron playbook as it allows you to bring the massed firepower of your warriors and destroyers on one part of your opponent’s army while only having to face half of his army in return. All of it depends on deployment and the relative speed of your opponent’s army- foot, hybrid, or all mech.
Here is an example of castling in action from a recent game: Mission is annihilation my opponent goes first and spreads himself out on the table, not a bad idea as it allows his scatter lasers to cover all over the table. However, on the right and center he has guardians on foot, along with an avatar to the far right, and a wraithlord to the left. Wave serpents and warp spiders to the center and side.
For my deployment I cluster up to the far right so my entire army is taking on the wave serpent and wraitlord- which I can throw wounds on with no problem thanks to the gauss weapons, effectively turning him into a space marine with a 3+ save- easy kill point.
The other wave serpent and warp spider will be on my phalanx in a turn or two, but the avatar and pretty much all the guardians are out of the game till later at best. Taking advantage of this deployment meant my full army vs. half an army.
The key here is that after your opponent has deployed (assuming they go first) take a moment to really look at their deployment for any mistakes or points that you can exploit.