I think it depends on your personal body metabolism. I bascially have one bag, a 10-degree rated WM Super Antelope that weighs 2# 12 oz. I did not need all that warmth for 9 out of 10 nights on my recent trip. One night it got below freezing and I was glad to have it. My body seems to simply shut down on producing warmth when I sleep, so for me, it is not too warm. On warmer nights I drape it over me. Do not rely on the bag "ratings", rather consider your personal needs.

My personal rule-of-thumb is "no bag over 3 pounds". I could get a 35-degree "summer" bag but that would only save a few ounces, unless I were to spend lots of money to get the high quality lighter weight bag. With a limited budget, I need the money to pay for gas!

Overall, probably 50% of my nights out are below freezing. My "shoulder seasons" nights are usually around 15-20 degrees. If you hike in a warmer climate, then, a zero-degree bag may be too warm.