4L30E AT transmission

Hi I know this is a little off topic for this newsgroup, but this group has people with expertise in about any field pertaining to vehicles. The 4L30E
is used in a couple GM cars, as well as the Honda Passport, Isuzu Rodeo and Amigo.
I need to change the AT fluid and filter in my passport with the 4L30E transmission. The shop manual says purchase a transmission fluid thermometer from you local parts counter before attempting this or serious damage may be done to your transmission. It has no dip stick, so fluid has to be pumped in through the drain plug or overflow check hole. Is it really so critical that you have to check the temp of the fluid before adding more. This seem like a lot of Bull to me, but I am not a professional like some of you guys.
Would like to hear what some of you transmission guys think about this. Do you know of another safe way to check it without thermometer? Charles
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