It helps somewhat, but I don't see it a necessity. The Silmarillion is a book that you have to read with some commitment and effort. If you read it before Unifinished Tales, you'll have some better background. But even then, the book is more than readable without this precursor. It's really up to you. I would just read Unfinished Tales, and if you're confused about some things, go back and read Silmarillion.