Steven, using a higher rated bag will save on pack weight, but also you might want to carry another set of light weight base layers to sleep in in the summer time. Many posters, myself included, don't like the feeling of bare skin on the clammy nylon of a sleeping bag. LW wool such as that from Montbell is not opressively hot, and breathes better than Capilene 1 IMO. I even wear thin socks to prevent my feet from contacting clammy nylon. Also, if you are sleeping on top of your bag because it is too hot, a base layer sometimes gives just enough warmth to avoid having to crawl in and out.
And in winter time of course, wearing a base layer in your bag adds a few degrees of warmth.
I can use a Montbell Super Stretch Down Hugger#7 rated at 50F down to 40F with a LW base layer.