Good Sweat?

*giggle* Okay those of you freezing' your tootsies off, I'm terribly sorry for this serious of questions... really, I am. but it's been in the 80s during the day, moderate hum-didity, and Chewie works up some sweat during his 30-45 minute workouts.

Is it true a horse sweats where they use the muscles? For example, if Chewie has a "bad ride day", ie head all over the place, nervous, hollow back, and takes off in the warm-up (without me on him) like a goober, his lower neck will sweat, and a little on his shoulders. When he's had a "great ride day", ie head down, bent at poll, round back, nice full trot, he'll sweat at the flank, stifle, sometimes rear quarter, and just to the sides of his tail around his butt. If there's any sweat on the front of him, it's way low on his shoulders near the elbows.

I had a lady tell me one time that his sweat marks after a ride are a good indication of whether he worked "well" or not that day. Is that true?