One of the most important things you can do is keep your insulation dry. It's just a basic responsibility to yourself. You'll never catch me just jamming a sleeping bag straight into a pack or next to a tent (if you're taking a tent, I'm presuming you might expect some rain?). So I always use a lightweight dry sack.
Added benefit of the dry sack is that it can be made to get your sleeping bag every bit as small as in a compression sack, but in a much more efficient packing shape. Make one roll on the top of the drysack, leave an end of it loosely open, and sit or kneel on the drysack. Your body weight will squeeze all the air out. Seal the sack. I buy the sacks a little bigger than I need for this step: "knead" and/or flatten out the sack into a tidy, flat, shapable brick. Way less of a pain to pack than a compression sack!
A 13L sea to summit fits my Summerlite and 3-season clothing.