Now all this has got me wondering why there is a rubber hose there at all -- as you can see in the picture above, it is metal line from the transmission and metal line with a quick connect to the cooler with a rubber length of 6" or so in the middle. Is that there for any reason other than to have something to attach the check valve to? If we are deleting the check valve anyway, wouldn't it be better to replace the rubber hose with a section of metal line with double flares at each end?
I think that if it is factory installed, it would be to allow engine/transmission movement without cracking the tubing or loosening the fittings.
I my 96 Buick Roadmaster 350 with 4l60E transmission it had rubber lines as the final connection, with crimped on hose ends. When I got the car it was 17 years old and 120K miles and they did leak some, though. The 07 van also shows lines with rubber in them, but they are long gone on ours.
In general, I like some flexibility in the lines as cracked cooler lines have always been an issue on some vehicles.
AFAIK, the only reason for a check valve is to keep the cooler(s) from draining back into the trans and needing to be pumped up before the trans is used. I put on on the van when I did the two cooler setup as it is a lot of volume in those coolers. I did not put one in the Buick when I did that one and it is also fine.
If you are worried about the pump up time without a checkvalve, you might want to put in a bypass thermostat as it will bypass most of the cooler oil and send it right back to the transmission, especially in cold weather when thick oil makes them pump up slower.