You do not have to pre-cook the chicken before putting it into a slow cooker; however, it is advisable to remove the skin because of the fat, unless you brown the breast first. This browning also gives some color and a better, richer taste to the breasts.
One thing I suggest (and my manual/guide suggests the same thing) is that when you put the breasts in, your slow-cooker should be on high for the first hour or so. This brings the food up to temperature faster and deters the growth of bacteria, which can develop in any meat or poultry if it is cooked at too low a temperatutre. I'm sure most crock-pot manuals/guides will give you the same advice. By the way, "Graptos" & "Tipnut" give some pretty good advice too.
Good luck & bon appetit!!