Pillcaster,
It is always in my vest or sling pack. I rely more on it for trout activity than bug/prey activity. If the water is below about 45 degrees I will spend more of my time fishing the seem waters or in tail outs where they can hold with out much energy output. once the water begins to approach 50 deg. I will start to fish more in the riffle /run waters. one big reason to have it is to know exactly when the water has gotten over 68 degrees and time to head to the local tailwater. actual bug activity can be regional as well. I know here in CT we will have midges on the water when the water temp is 39 degrees. Rarely will the trout be rising to them on the freestones but you can get some good surface action on the tailwater then as well. In my 40 years of fly fishing I have found the best way to figure out what is hatching is to shake the trees/brush , seine the water to see what is there.

steve