Last July I did an overnight in the mountain bridge wilderness along the SC/NC border. It was warm enough I didn't use the under quilt, but I took it. I was a little cool underneath for a part of the night, but nothing uncomfortable. I'd say always take it, if you have to, you can get up at 2am and string it up. Mine involves clipping on two s-biners and thats it.
I've been to a couple of places where I was thinking "there's no way I'll need the UQ" but have been glad to have it. Drove up to the Blue Ridge Parkway with some friends in August - it was 100 degrees in Columbia SC. We're all in t-shirts and shorts. Got to the mountains and its cloudy and misty, and about 60 degrees with wind. We put on everything we had and we all got in the tents/hammocks by 630 because we finished eating and between the mist and wind were all freezing. Without the UQ it would have been a miserable night.