Under 65 degrees, cover the knees. Especially if the sun isn't shining and/or there is a wind. This maxim did not originate with me. I read it about 20 years ago (Alex Steida or Davis Phinney or somebody else), and have followed it faithfully, although I might fudge 5 degrees if the sun is bright and there isn't any wind.
One thing that did originate with me, however, is having three layers on your chest under 65 degrees, with any wind at all, in the winter. I just got in from my standard 31 mile ride. It was 58 degrees, sunshine, wind out of the northwest. I wore shorts, tights, base layer, jersey, and wind vest. I was not too hot. Arriving home I had ginger tea and 500mg vitamin C, also stretched. Don't wait for the sniffles before taking action.
On another note, I just received notice of a race in early February. Based on yesterday's tt practice, I don't think I'm ready for prime time in three weeks. But I haven't read all the details, and haven't had a couple more practices, so I'm not ruling it completely out.