I used a Trailwise Slimline bag rated at zero degrees for years, for all conditions, and it worked fine, but was occasionally too warm. I get cold easily, so I was sometimes glad I had the extra warmth. A good quality down bag rated at about 20 degrees is probably as close as you'll get to an all-around bag.

What you're using for shelter makes a big difference. If you like to sleep out under the stars, you'll need a somewhat warmer bag than if you're in a tent.

Also, many budget-priced bags are rated very optimistically. Check the overall loft. To my way of thinking, a zero-degree bag will have about 7" of overall loft, and a 20-degree bag will have about 5.5" of loft. I'm talking only about mummy-type bags here, with a hood and draft flap.