I have been sewing sugar glider items since around 2003. I have seen it done both ways. Personally, I wash my fleece as soon as I get it in cold water and mild detergent.

I will never forget walking through a local fabric store and seeing a little boy wiping his nose with his hand and then wiping his hands of various bolts of fabric. I reported it when I got to the register, but seriously I doubt they did much about it. I don't always sew everything I buy right away so I would rather wash everything before I cut or store instead of possibly introducing gross things to my fabric hoard.

Also, sometimes fleece will have wrinkles or, if you get fleece at the end of the bold, it can be stretched making it difficult to properly cut and measure. Washing before sewing can reset the fleece and make it easier to work with.

Oh my, that is gross! Where was his mom looking? I might start washing twice.