It may depend on you project. Slipped stitches make a nice edge for a project like a scarf that will not be seamed. But even then it is a matter of choice and the look you want.
But if you are going to seam the piece it is a choice not so much of what looks good, but what you like to work from for seaming. Some people swear by slipping stitches for all their seaming. I personally don't do that. And I took a finishing class and the teacher said, "Don't slip if you're going to seam." So people have individual preferences. You'll have to learn what works best for you.