There is a special non-documented CMP protocol (Cluster Management Protocol, sent always through VLAN1 on trunk, similary to CDP) used for Master - Member communication in the cluster.
It's using special IP addresses derived form switch MAC addresses. (See NAT commands in switch config when enabling a cluster). I've noticed a problem when using ACL to HTTP access on switches in the past - the CMP IP addresses have to be allowed (I don't know the exact interaction between CMP and HTTP - I think it's a bug).
My experience with the @esN community strings is that they are never used when each switch has it's IP address assigned. You can use a standard SNMP management in this case. BTW, neither HP Open View nor CiscoWorks do support clusters.
So my personal opinion is the only reason for using clusters is the possibility to see several switches on one browser screen while connecting via HTTP.
But I recommend to configure an IP address on each switch and remove the @esN community strings from the config.
If you can use CiscoWorks, use Device Manager -> CiscoView -> StackMaker instead of Cluster and you'll have the same functionality through CiscoView without any problems with CMP and community strings.