GW -
I try to install mine around mid October when temps are consistently cooler. That being said, I don't usually get around to it until about Thanksgiving.
Remember, you don't actually "need" to have one. It is more for personal comfort and preference. So, when you install it is up to you.
I didn't go with the Eric B inserts because they are either "in" or "out". You either have the front of the truck closed or open. Others have had great luck with them and I have no doubt they are a fine product. Personal preference again... .
With the Mopar blanket, I have the opening in the front. It has 4 velcro flaps. By utilizing one or all of the flaps I can have it closed, partially closed, or open. If I see temps running a little lower or higher that I care, I can stop, open one or all of the flaps and continue to march. Takes all of 30 seconds. It just gives me more options I think.
Best -
Steve