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.