I don't know what the official word on this is.. there are differing opinions. I would probably place repeater(s) 1/3 to 1/2 way between hub and farthest device on your mesh filling in any "gaps" around the hub. The switches etc should also be acting as repeaters. If you have any Z-Wave locks then placing a repeater near those may be helpful if you are experiencing any kind of lock instabilities. Also keep in mind the 3d nature of this - a repeater on another floor can be close to the hub so may be helpful in getting through walls/ceilings etc.
What type of repeater to get? The Ring Alarm Range Extender is inexpensive and comes with a battery backup so you can detect power outages..
Pairing can be a little bit of a pain - you may have to set classic inclusion mode. Here's an older thread post that discusses how to do it:
