It’s not going to hurt to poke some holes to allow any fluid out, so go ahead and do it. It may not be needed, but shouldn’t hurt anything. A skull mount with velvet antlers isn’t the best project for a first DIY attempt. You’re looking for two opposite results on the same piece, preservation of the velvet, and complete removal of all other tissue. To me, the antlers need to be pinned, removed, and treated completely separately. I’d also suggest not dragging the deer by the velvet covered antlers.