If everything is pre-cooked, from a food safety point of view it doesn't matter - you could eat it with no cooking and be just fine. Taste, on the other hand would be improved by cooking, so...
The biggest difference between the two setting is how long it takes for the whole pot to get up to the set temperature. One of the key things it to minimize the opening of the lid, because that releases a whole lot of heat energy in the form of steam. It takes at least 15 minutes to get back up to temp after opening the lid! That is why so many recipes start on high and then go to low after a few hours.
From the Crock-Pot website (or at least old references to it, since their current website at http://www.crock-pot.com/CustomerService.aspx?id=faq&fgid=44#tabs doesn't seem to have it any more), basically "High" takes four hours less than "Low".
Q: What’s the difference between "Low" and "High" cooking?
A: Both "High" and "Low" stabilize at the same temperature; it is just a matter of how long it takes to reach the simmer point. Once food reaches the simmer point, total cook time is dependent on cut and weight of meat to reach the point of maximum flavor and texture potential. Most dishes can be prepared on either "High or "Low."
Q: What are the typical cook times for Crock-Pot® Slow Cookers?
A: Typical cook time for Crock-Pot® SlowCookers to reach simmer point is 209°F:
Low: 7-8 hours to reach the simmer point
High: 3-4 hours to reach the simmer point
Q: How do I convert cook times between "High and "Low?
A: Below is a conversion chart to illustrate the comparative cook times for "High" and "Low"*
3 hours 7 hours
4 hours 8 hours
5 hours 9 hours
6 hours 10 hours
7 hours 11 hours
8 hours 12 hours
*** It is not recommended to convert recipes with cook times less than 7-8 hours on "Low" or 3-4 hours on "High."
So it depends on the length of time recommended in the original recipe as to how long to cook it on High.