It's a little hit and miss. If the stack is nice and high, you can get away with having the mics on your shoulders if you're close enough. But given a preference between have a wider spread (shoulders) and having them up higher (hat), I'd choose hat 100% of the time. There's two reasons...first is getting direct sound from the source (the lower the mics, the more chance you'll get muffling because sound isn't direct but bouncing off of bodies and other things) and second is your mics will be at mouth level for all of the chatty folks and since the mics are less likely to be getting direct sound from the speakers, the talking will be more predominant, whereas when you have a higher SPL from direct sound, the talking can fade a little into the background. Sometimes it only takes a foot of actual distance (shoulders to hat distance) to make a pretty large difference in the final result.