Personally, I wash in hot and dry in the dryer (I used to air dry, but my little one gets sick easily, so all steps in cleaning are designed to kill off germs), so when making stuff out of fleece, I use that protocol - wash in hot; dry in dryer.
Alternately, if I regularly washed fleece in cold and dried on high, then I'd use a that same cold water & high heat dryer protocol before sewing. Else what are you going to do? Unhook your washer and reattach to hot for one load? I suppose you could go crazy with it and boil the fleece on your stovetop (I've done that before - when they had giardia) if you wanted to be super certain it won't shrink more. But the texture flattens afterward; it's not as soft and thick.
I'd vote for tossing the new fleece in for a couple of regular loads before cutting & sewing and call it a day.