Here is a great article provided by the USDA Food Safety and Inspection Service. I’m sharing it with all of y’all because I think it will answer some of your (and my) questions.
Opening the front door on a cold winter evening and being greeted by the inviting smells of beef stew or chicken noodle soup wafting from a slow cooker can be a diner’s dream come true. But winter is not the only time a slow cooker is useful. In the summer, using this small electrical appliance can avoid introducing heat from a hot oven. At any time of year, a slow cooker can make life a little more convenient because by planning ahead, you save time later. And it takes less electricity to use a slow cooker rather than an oven.
Is A Slow Cooker Safe?
Yes, the slow cooker, a countertop electrical appliance, cooks foods slowly at a low temperature—generally between 170° and 280° F. The low heat helps less expensive, leaner cuts of meat become tender and shrink less.
The direct heat from the pot, lengthy cooking and steam created within the tightly-covered container combine to destroy bacteria and make the slow cooker a safe process for cooking foods.
Safe Beginnings
Begin with a clean cooker, clean utensils and a clean work area. Wash hands before and during food preparation.
Keep perishable foods refrigerated until preparation time. If you cut up meat and vegetables in advance, store them separately in the refrigerator. The slow cooker may take several hours to reach a safe, bacteria-killing temperature. Constant refrigeration assures that bacteria, which multiply rapidly at room temperature, won’t get a “head start” during the first few hours of cooking.
Thaw Ingredients
Always thaw meat or poultry before putting it into a slow cooker. Choose to make foods with a high moisture content such as chili, soup, stew or spaghetti sauce. If using a commercially frozen slow cooker meal, prepare according to manufacturer’s instructions.
Use the Right Amount of Food
Fill cooker no less than half full and no more than two-thirds full. Vegetables cook slower than meat and poultry in a slow cooker so if using them, put the vegetables in first. Then add the meat and desired amount of liquid such as broth, water or barbecue sauce. Keep the lid in place, removing only to stir the food or check for doneness. 
REMINDERS:
- Fill cooker no less than half full and no more than two-thirds full.
- Add desired amount of liquid.
- Keep the lid in place.
Settings
Most cookers have two or more settings. Foods take different times to cook depending upon the setting used. Certainly, foods will cook faster on high than on low. However, for all-day cooking or for less-tender cuts, you may want to use the low setting.
If possible, turn the cooker on the highest setting for the first hour of cooking time and then to low or the setting called for in your recipe. However, it’s safe to cook foods on low the entire time — if you’re leaving for work, for example, and preparation time is limited.
While food is cooking and once it’s done, food will stay safe as long as the cooker is operating.
Power Out
If you are not at home during the entire slow-cooking process and the power goes out, throw away the food even if it looks done.
If you are at home, finish cooking the ingredients immediately by some other means: on a gas stove, on the outdoor grill or at a house where the power is on.
When you are at home, and if the food was completely cooked before the power went out, the food should remain safe up to two hours in the cooker with the power off.
Handling Leftovers
Store leftovers in shallow covered containers and refrigerate within two hours after cooking is finished. Reheating leftovers in a slow cooker is not recommended. Cooked food should be reheated on the stove, in a microwave, or in a conventional oven until it reaches 165 °F. Then the hot food can be placed in a preheated slow cooker to keep it hot for serving—at least 140 °F as measured with a food thermometer. Last Modified: February 13, 2009
Resource – USDA website