You have to leave them enabled. The assist fan profile 1 has to be set to fan off on both of the assist fans. This will keep them off while the pc is running. They will come on from time to time when the pc turns on or off. Also, when you open a program the fans will come on then they will go off.

Dicehunter wrote:
The 2 x assist fans on the Sabertooth Z87 are really loud, Are they really required or can I remove them and put the plastic covers back on ?

They really helps to cool down PCH and Vcore but they are loud! Especially when they spins at full speed. Unfortunately BIOS does not provide freedom to set these fans on quiet because isn't possible to set lowest speed under 60% or, simply, setting Q-Fan is not enough.

What you can do is:
Turn them off in BIOS (I don't recommend this if you have high ambient temperatures or you load your PC with heavy tasks)
Install and use AI Suite 3 (Thermal Radar) and set them like in this pictures




As you can see, I've set them to Auto Fan Stop and in 90% of time they are off