I am totally with Greg on this one. I usually layer in a bag when it's cold. Typically, when it's really cold (during 3-season backpacking) that includes long underwear, warm and dry and clean sleeping socks, the clothes I've hiked in, a down jacket and a balaclava. Sometimes my breathable rain jacket, too. Inside a down bag. And I've often worn synthetics inside a down bag. If it's a problem, I've failed to notice it. I HAVE noticed that I find layering is warmer.
Like layering clothing alone, it might take a little practice to figure out what works best. A good fit between all layers is important. I prefer to wear my insulation rather than lay it on top of my bag, because I move when I sleep. I don't wear soaked clothing in a down bag. I avoid getting sweated up just before or during bedtime.