There are three noderoles: master (or primary), backup, and standby. It should not be none in stacking.
it is recommended to have master-capability on, this will help when the master switch goes down, backup will becomes master and the other standby node which has master-capability on will becomes backup.
it is always recommended to have latest & stable software to be install on switch. For X440 stack, I would like to recommend to have EXOS 15.3.1.4 patch 1-31 software installed.
You can download the software from esupport.extremenetworks,com.