Not proofreading can be quite embarrassing. Even if you think you've been careful, proofreading will often pick up mistakes. If you have time, just like when editing, it's best to step away from your work and come back to proofread after bit of time has passed. It helps avoid our tendency to see what we expect to see, rather than what is actually there, and you'll pick up mistakes you might have overlooked at first glance.