I don't usually cover much of anything at this time of year either, I figure if it's over it's over! But, last spring I did find myself using some sheets for some things I didn't want to lose. I did that when the forecast was for anything from say 37 degrees to low 40's because it seemed like I was getting frost here and there at those temps.
I think freezing is more likely with calm, clear air and no wind; the kind of weather you get after a front moves through. If there is a cloud cover with breezes I think you're safer. Also, if your garden is in a lower lying area, as some of mine is, you might be in greater danger since cold air is heavier. If your plants are up against a building, you will probably be safer.