You cannot run an automatic without a trans cooler in post 64 vehicles. The only ones that could were 6 cylinder powerglides in GM vehicles, they had enough airflow and such limited power it could be air cooled. Dont try it with a C4, you will overheat it in short order.

One nice thing about having it in the radiator is it warms up faster in cold weather, transmissions dont like too much heat but they do like some. Its not absolutely necessary to have it in the radiator, but you need one. If you do mostly highway driving you dont need a fan on it especially if its getting airflow from the fan behind the radiator. If you have a high stall converter or sit and idle in traffic often, a fan on the cooler is a great idea. My toy has a 4200 stall in it, it still drives sorta normal but it does slip under 2500 or so. I have a HUGE cooler on it, usually installed in heavy trucks and a large fan all its own to cool it. The radiator doesnt have a cooler in it, so it needed its own fan. A trans temp guage when running a high stall like that is a great idea if not mandatory to ensure it isnt getting too hot.

Heat kills a trans, and unless you live in Alaska or some northern states or canada you dont have to worry about it being too cold. If you see below zero for extended periods, you need to have some heat for it. The fluid becomes really thick and you can blow seals and even servo covers off in extreme cold. Its gotta have some heat if it sleeps in the cold.