I'm actually headed out to the Foothills Trail to thru-hike it this coming Sunday to Tuesday (though we may extend that to Wednesday if I find I can't keep the pace needed), and I'm leaving my underquilt at home. It's looking like forecasted mid-70s for the nightly lows, which means that I can get away with shoving my pack under my torso if I need extra bottom insulation (I assume that the forecast is up to 10 F off). Note, though, that I use a synthetic top quilt (which retains some insulative value tucked under my shoulders and hips compared to a down quilt) and am a very hot sleeper.
If worse comes to absolute worst (say, it drops to 55 F and rains), I can take my poncho and suspend it underneath as a makeshift underquilt with forest duff shoved into it and then lay on top of my space blanket in the hammock to prevent evaporative heat loss in that direction.
Anyway, I think that--as long as temperatures stay above 65 F--you'll be fine just shoving your pack under your torso if you sleep warm. If not, 70 is probably the cutoff.