One thing last year's nasty case of plantar fasciitis taught me is that when we sleep, our feet relax into the pointed toe position (why night splints are recommended for PF). It really depends on how you sleep. If you sleep on your back stretched out, you will take up a couple inches more than your height. In other words, anyone over 5'4" will not have enough foot room in a 5'6" bag. For those who always sleep curled on on the side, it might not be a big deal. People change, though. I've always been a side sleeper, but the last few years I have shifted to occasional back sleeping. I'm 5' 3 1/2" and my toes just touch the bottom of a short Western Mountaineering bag if I stretch out full length. If I were any taller, I'd definitely want a medium bag.
And always try a bag on while you can still return it; girth is an important size aspect of a sleeping bag, as is an individual's tolerance for how snug a bag can be tolerated.