Unless your recipe specifies that you need to brown your meat before placing it in the slow cooker, you do not need to brown it first.
The one exception – which affects looks only, but not taste – is ground beef. Ground beef that is cooked in the slow cooker will appear gray in color rather than the nice brown you are used to. But this is strictly a step for appearances only.
Some recipes do call for pre-browning, particularly chicken. But this step is not required unless your slow cooker recipe calls for you to do it prior to placing in your slow cooker.