Yes! Searing a roast (or other cut of meat prior to baking, roasting, or slow cooking) is a most important step. Why? Searing creates a caramelized exterior which holds in the juices and flavor of the meat while it's being cooked. It also adds extra flavor. Never omit this step.
I love roasts in my slow cooker. And the flavor of prime rib roast simply cannot be beat. I like to use a "purist" approach to this cut because it has such a wonderful, buttery flavor it would be a shame to cover it up with lots of other things.
I sear mine in a skillet with a mixture of one part butter and one part olive oil (you can use 2 tablespoons of each). Season the roast with salt and pepper on all sides, then heat the skillet on the flame first, then add the oil and butter and allow it to heat THEN add the roast. This is how you get that wonderful and important sear. Hot pan, hot oil, then the meat. Turn the roast over when each side has seared, getting that caramel color all around.
Place the seared roast into the slow cooker along with 2 garlic cloves and 1/2 cup beef broth or beef stock. Cover and cook on high until the roast is done to your liking. Most people prefer it medium rare, but medium is also good. You'll need a good meat thermometer for this (the type which pokes down into the meat). For rare, look for an internal temperature of 120 degrees Farenheit (40 Celsius), medium rare 130 degrees Farenheit (55 Celsius), and medium 140 Farenheit (60 Celsius) degrees. Keep in mind that the roast will continue to cook a bit after it's removed from the cooker. Place the roast on a rack or platter and allow to rest for 15 minutes before carving. This is another very important step in preparing a good roast. Allowing the meat to rest causes the juices to flow back into the structure. Otherwise, when it's carved - all the juices will run out and you'll have a dry roast.
You'll undoubtedly get lots of answers with great recipes calling for all sorts of vegetables and herbs and they'll be wonderful - don't get me wrong. But as I stated above: prime rib is known for its distinctive and mouth-watering flavor and I prefer to enjoy it with only salt, pepper, butter and garlic to enhance its flavor.