+1 for everyone is different. I would not need anything underneath at 75*F. Neither would Thing1... she sleeps indoors at 75*F without insulation under her hammock every night!
Essentially, you need something to combat CBS (Cold Butt Syndrome), just in case you experience it. Perhaps you turn in wet from rain, or you catch a cold, or you have not eaten as well as you should. Maybe it's a windy night. Whatever the reason, you might need a wee bit of insulation.
You could use something like one of the sit pads I made recently. It's not as heavy or thick as CCF, just a 2'X3' nylon covering with a thin layer of any kind of insulation inside. Normally, it will be your sit pad. When you feel a little cool on the bottom of your hammock, just put the sit pad under your torso, inside the hammock. It needs to be something flimsy enough to conform to your backside, preferably not as stiff as CCF. You can adjust the size to suit the material you choose.
You could use one of 2QZQ's $25 UQ Protector. You don't have to have a UQ to use the Protector. It's a full length silnylon weather shield, so it will block the wind and wind blown rain. It's a vapor barrier, which might not be a good thing in your climate.
Looking forward to hearing about your experiments!
"We must, indeed, all hang together or, most assuredly, we shall all hang separately." - Ben Franklin