A certain part of me thinks that if Sir_Godspeed is asking a fluff question then there is little that I can offer. Also I am not that good at answering fluff orientated question as most of GW's fluff seems to offer very little depth. I will a have ago at this though just to see how wrong I can be. Bear in mind all that I know about Necrons is what is provided by the fifth edition Necrons Codex.
All of this makes me wonder if any Necron lord has every decided to deliberately destabilize a planet or sector by insitgating cult activity
Due to the 'many millions' of Tomb Worlds that exist and the autonomous nature of their rulers there must be a chance that one of the rulers has tried to instigate some form of subversion in order to destabilise a planet.
There is a sentence on page 9 of the fifth edition Codex that states - '...At others, every possible underhanded tactic is employed, from piracy and deception, to assassination and subornation.'
For me the phrase 'every possible underhand tactic' means that the use of 'cults' can't be ruled out.
I would guess that if you have lived for over sixty five million years you might be willing to play a few games with the lesser beings.
Overall though I can't believe that a race that is as technologically advanced as the Necrons would bother with such a low tech approach as instigating a cult.
Page 25 of the fifth Codex lets us know that the Necrons made a slave world of hive world Aryand and page 26 tells us that Overlord Janzikh used mindshackle scarabs to infest and the control the military forces of Burr. These are the kind of actions that I think suit the Necrons best.
or if there are parts of the Mechanicus, for example, who actively look to the Necrons specifically to worship, or anything like that.
On the other hand worship of Necrons by humans is something I can totally believe in as we have a proven predilection to worship. I don't know too much about the Mechanicus, but I can understand that some of them might view the Necrons as the ultimate 'mod'. I think I could see this leading to some sort of worship.
Edit: missing words added.