"The more you remove dice from the game, the more you will win consistently."
The Aegis Defence Lines I've seen at Leicester All Scars are normally deployed where their models start rather than finish the battle. Deploying an Aegis outside your deployment zone is statistically better than standing behind it for a whole game as you can consolidate behind it to claim an objective.
The Aegis needs from my observations to be roughly 6-12" further up; even if the initiative is stolen, you're still getting the benefits of the Aegis 90% of the time and when it's not stolen, your army is in a better position game long and importantly you are giving your troops an opportunity to benefit from using their weapons at a shorter range. For example, Guardsman either need to expose themselves outside of 4+ cover save from the start of the game to have a more effective threat range in order to rapid fire or are a static 24" threat.
Therefore put the aegis where you want to be Turn 1 and more often than not, push it a little bit further up so you're not limiting your movement Turn 2.