If the APs are in the LAN the default should work. If the APs are connected via a WAN/VPN with reduced MTU you'd need to use the minimum MTU which is supported on the link. I.e. I've a AP3825 @home = cable modem provider and I've a VPN tunnel with a firewall to the HQ. In that case I've used 1300 as my DSL and VPN take away from the max. of 1500.