If you can, do a search for a local Diesel Performance shop. They can help you out with you decision to get guages. If not, do some reading on
www.turbodieselregister.com - in fact you should visit there every now and then anyway, you can learn a lot from reading the forums there.
Cool down is important if your running hot. Just remember that extended idling is also considered "undesireable" by Cummins. No specifics were ever given, but theres been a lot of speculation as to why.