Should timing data be gathered for every iRule invocation?

it seems okay here. you may open a support case to check.

root@(B6900-R69-S40)(cfg-sync Standalone)(Active)(/Common)(tmos) list ltm virtual bar
ltm virtual bar {
    destination 100.100.100.41:80
    ip-protocol tcp
    mask 255.255.255.255
    pool foo
    profiles {
        http { }
        tcp { }
    }
    rules {
        qux
    }
    source 0.0.0.0/0
    source-address-translation {
        type automap
    }
    vs-index 3
}
root@(B6900-R69-S40)(cfg-sync Standalone)(Active)(/Common)(tmos) list ltm rule qux
ltm rule qux {
    timing on
when RULE_INIT {
  log local0. ""
}
when CLIENT_ACCEPTED {
  log local0. ""
}
}
root@(B6900-R69-S40)(cfg-sync Standalone)(Active)(/Common)(tmos) show ltm rule qux

------------------------------------
Ltm::Rule Event: qux:CLIENT_ACCEPTED
------------------------------------
Priority                    500
Executions
  Total                  1000.0
  Failures                    0
  Aborts                      0
CPU Cycles on Executing
  Average                 51.6K
  Maximum                170.9K
  Minimum                 15.8K

------------------------------
Ltm::Rule Event: qux:RULE_INIT
------------------------------
Priority                 500
Executions
  Total                    0
  Failures                 0
  Aborts                   0
CPU Cycles on Executing
  Average                  0
  Maximum                  0
  Minimum                  0