Were people in past centuries (we're talking 18th century and before) aware of how poorly they must have smelled if they weren't bathing every day? Or did they just not notice because everyone reeked of sweat?

This sort of addresses your question, but has nothing to do with history -- so mods, feel free to delete :) But your question actually touches on a very interesting active area of study in neuroscience, and that's neural adaptation. Scientists have noticed since the 19th century that certain stimuli undergo extinction -- that is, you'll stop to notice them. When you encounter a new scent it's obvious. But very predictably, the smell will fade away -- to the point that you literally CANNOT smell it. We experience it with all the senses, but smell is the most dramatic. The classic example is when you get back to your house from vacation -- you've given your brain enough time to "unadapt" and you notice the smell of your house again, but rapidly your brain will get used to it. Only in the past few years have been begun to study the process by which this happens, and it actually involves changes in brain signalling (though cGMP, though all of this stuff is well above my head). The evolutionary reason appears to be to protect against too much stimuli. All of this is to say -- they probably didn't notice too much, because brains will block out smells pretty rapidly.

Sorry for the non-historical tangent.