I will tell you this (which I've posted elsewhere): Last fall, just two months into my first foray into vaping, I started experiencing pain in my feet whenever I walked for five minutes or more at my normal (brisk) pace. It got worse over the next few weeks to the point where after I got to my destination my feet were killing me. Then the random but frequent heart palpitations started, and that freaked me right the hell out.
At the time I thought maybe it was something akin to nicotine poisoning. I didn't feel sick or dizzy or anything, but I thought maybe I was vaping too much or the liquid was too strong (at the time, 18mg). I tried ordering much lighter juice but I couldn't vape it; no throat hit = no point for me. I ended up going back to smoking for several months because I thought I just couldn't vape.
But then I ran into someone on another forum (on Fark, of all places) who was experiencing the same problem, and he had nailed the source of the problems down to cheap, bad juice from China. Cheap juice from China was the last thing I was vaping regularly, bought from a local vendor who drop shipped from China. (GreenhouseOne, but see below.) Everything just kinda clicked: IT must have been that juice, not vaping in general.
So I went and ordered some new hardware (was trying the new tanks at the time) plus some new juice from Vaperite, and just to prove the point, I ordered the strongest he had, 26mg. Long story short, that was 4 months ago and I've been vaping 24-26mg the whole time. I also ran into someone here just a week or two ago who had the exact same problems, and who had also been vaping juice from the exact same vendor I got my cheap Chinese juice from. So, despite the small sample size, I'm convinced that it was definitely the juice.
So yes. Made in the USA does make a difference. Note that this is not an indictment against all juices from China. Lots of people have been vaping various Chinese brands like Dekang for ages without issue. It seems so far to have been just this one juice (utterly generic, no manufacturer listed, no identifying markings on the bottles or labels). Still, that was more than enough to make me extremely cautious about who I get my juice from, and now I will only buy hand-made juices made in North America with locally-sourced ingredients, with the exception of well-established foreign brands people haven't had any problems with, like Dekang.
Also, please note that this is not an indictment against GreenhouseOne, either, just whomever they get their juice from. GHO have been good to me, I've ordered hardware from them without any issues at all. I'll just never order their juice again and would not recommend anyone else does, either.
Just my anecdotal $0.02.