Sounds like a touch of paranoia. Signs of skunks include scratch marks on the landing board and the bees being stirred up seemingly without cause. There are quit a few deterrents for skunks from tacking a carpet nail strip to the landing board to rolling up chicken wire into a tube and putting it in front of the hive, even having your hive stand on cinder blocks helps as it exposes the soft underbelly of the beast to the bees. Robbing mainly occurs in the fall during a dearth. To get robbing going in the spring with a flow would take some effort on the beeks part if it can be done at all. Personally I have never seen it in the spring. It would not hurt anything to keep your entrance small all year except perhaps honey production. In the wild most hives have very small openings. The larger opening makes for less traffic jams and more traffic. So it is really up to you, give it a try, the bees don't care either way. The only other invader I can think of are mice and they invade in the late fall.