While there are some standards for testing sleeping bags, every manufacturer seems to do their own tests and do it in their own way. If you can, learn how they do it first as this can help you make an educated guess. If they don't advertise how they test the bags, then maybe you're looking into the wrong brand. The standard the MEC follows is set out by the ASTM and I've had very good luck with their bags. I believe The North Face does too but I can't find anything proving that right now. I'm also a big fan of TNF bags.

For me I used a +20F bag nearly year round but it's part of a modular system with layers which can be added or removed to suit conditions. At most I'll have 6 layers and at minimum 2 or 3 depending. I've tried and liked about 6 different combinations for different seasons and temperature ranges and I'm still coming up with new combos as I go. I'm hoping for some REALLY cold weather this winter so I can test it out in -40 or worse with just a tarp shelter.

Every time the first big cold snap hits and I see guys at the local outfitter buying -40 sleeping bags which are bigger than my whole pack, I laugh.