I use a 20* F bag (Western Mountaineering Ultralite) from April through October with no problems. I'm a cold sleeper, too. It's probably equivalent to a 15* bag with other brands. The draft collar feature (which you can pull snugly above your shoulders to keep cold air from hitting your body) is, IMHO, priceless. With it snugged down, I can keep the hood relatively loose (over my fleece balaclava) without having icy air flowing onto my body!
During our occasional summer hot spells (I live just east of Portland), I've often started the night on top of the bag. Later in the night I either pull it over me with the zipper undone or use it as a quilt. Usually by morning I'm inside with the bag partially zipped up, especially if I'm camping up high. With the big variation in temperatures we have here (99* one day and 55* the next, depending on the direction of air flow), it's important to have a full length zipper for ventilation!
At the other extreme, my fall trips always include a high-altitude trip in northern WA in early October to admire the bright gold of the alpine larch. It gets really cold up at 6-7000 feet at that time of year!
As Elizabeth says, girth is all important. Measure your shoulder (not chest) girth over your arms while wearing all your insulating clothing, and compare them to the bag's specs. While you don't want a lot of dead air space that your body has to heat up, you also don't want to be compressing the insulation of either the sleeping bag or your puffy jacket on a cold night. If you're in a store, insist that they let you try on the bag, and take that insulating clothing with you to put on (with the weather we've been having, you may need to wear it to the store anyway). If you order online, try out the bag as soon as you get it and return it if it isn't right for you. If you're just under the borderline between bag lengths, it's also important to try out the bag. Some have more room to stretch your feet out than others. If you're just over the borderline, get a long bag. In freezing weather you need room for your water filter and your stove fuel canister (if you use one) in the foot section.
My fall trips always include a high-altitude trip in northern WA in early October to feast my eyes on the bright gold of the alpine larch. It gets really cold up at 6-7000 feet at that time of year! The Ultralite, when combined with an adequate pad and my insulating clothing, has kept me warm down to 10*F.
That being said, if you're a normal sleeper as far as temp goes, and not a cold sleeper like me, and you don't plan to be out past mid-September, a 25* or possibly even a 30* bag combined with warm clothing and a sufficiently insulated pad would probably do the job.
To reiterate: Full-length zipper for ventilation. Draft collar for cold weather. Sufficient girth to wear your puffy insulating clothing inside.