When slow-cooking something new, I experiment with the heat settings, ala America's Test Kitchen. I haven't noticed a difference in most things, but when slow cooking tendon for inclusion in pho, the longer the better BY FAR!
For bone broth, nothing has effected the broth for me except for roasting ahead of time. Incidentally, I just wrote a post about slow cooking for chronic pain today.
It's a common misconception that HIGH and LOW refers to different cooking temperatures. Both HIGH and LOW cook the food at the same temperature but a HIGH setting takes less time to reach that temperature while a LOW setting brings up the temperature of the food slower. On HIGH, the food reaches the simmer point of 209 degrees in 3-4 hours, while on LOW, it takes 7-8 hours to reach 209 degrees. For more info, see here: http://www.crock-pot.com/CustomerService.aspx?id=faq&fgid=44
I usually cook my bone broths on high for 24-hours to leach out the maximum amount of nutrients.
I use the low setting, mainly because my crockpot offers only four options: 4 or 6 hours at the high setting, or 8 or 10 hours at the low setting. I choose the 10-hour settings every time and usually do a 10-hour one overnight then 10 hours while I'm at work all day.
That works well for me. I haven't made comparisons to a batch cooked on high.
I have high, low, and keep warm settings on my crockpot. I find keep warm is fine for overnight and longer cooking times. It is warm enough to keep the food safe and slow enough to suck all the goodness out of the bones.
Some chef recommends using a (specific type of) pressure cooker for making stock, so higher temperatures are not necessarily a bad thing, but aesthetically if the liquid doesn't boil then that results in a clearer stock.