I think if you are looking for general guideline, you should be look for
warning or worse events. Otherwise, it depends on what you are looking
for, how you have configured various thresholds. Usually, there are two
types of thresholds in Operations Manager e.g. Volume Full Threshold and
Volume Nearly Full Threshold. When "Nearly" thresholds are breached,
warning events are generated and when next threshold is breached error
events are generated.
You can make Operations Manager send traps for all events to your
3rd-party site monitoring tool as well. So, it is upto you which tool
you prefer.
I don't think there is any guideline as such for any of these things.