InterfaceTable question

All,
 
I am new to SNMP and its workings, thus this may seem a very simple question
with an obvious answer to the majority of you.
 
But, how often, if ever, should the counters in the InterfaceTable be reset?
Are they counters that should never be re-set or should they be re-set when
a device is powered off and powered back on?  Or are they dependant on a
Network Management Tool and how often that is re-initialized?
 
Many Thanks
Stuart