30 Gallon reef cube build 100% DIY

[Tyfighter]

I used to put egg crate on the bottom of my Africans cichlid tanks which was pretty common practise back then. The idea was the the rocks would sit on the egg crate instead of just on the sand or totally buried and sitting on the glass. It also provided a stable base since the cichlids really liked to dig in the sand which could cause the rocks to move/collapse.
As long as your rock structures are stable I wouldn't really worry about adding egg crate.