Yes, they do come pre-greased. All sealed ball bearings do come pre-greased.

However, the grease used in these and all bearings by all manufactures is not good enough, looks like Vaseline, is not water proof, and is not up to the task of preventing water and mud from entering the bearing, even just from washing, and especially not from a pressure washer or riding.

You should repack all sealed and unsealed ball or needle bearings and even bushings from any manufacture with marine (water proof) grease when used in a dirt bike application.

The grease used by all of the bearing manufactures is not as water proof as a good marine grease. Bel-ray water proof grease is the best. It is very tacky, water proof and stays in place,. I always remove the built in seals on the bearing, clean out the old grease with a solvent (like mineral spirits, WD-40, carburetor cleaner, Gun Scrubber, or brake cleaner), repack the insides completely with Bel-ray grease, and reinstall the seals. I also pack grease between the bearings and the external supplied seals as well as inside the lip of the seal itself. All seals should be lubricated to prevent the inner lip from burning up and allowing water and dirt to pass. The extra grease on the seal and between the seal and bearing helps form a barrier to water and mud getting to the bearings.

You will be surprised how easy it is to remove the rubber seals on these bearings and put them back in without damage. Use a small pick or micro screw driver. The 90° side of a scribe works very well. Carefully pick the outer edge up on the seal and it will pop right out. Do this by poking the scribe into the edge of the seal lightly and pry up gently.

You should do this to every bearing (new or used) on the bike. It should be done wither installing a new bearing, getting a new bike, getting a used bike, and periodically about every six months of riding on installed bearings. This applies to the steering head, wheel bearings, swing arm, shock, and link bearings. It may sound like a lot of work, but it isn't and it is a lot easier and cheaper then replacing bearings and seals

When I get a new bearing or a new or used bike I take it all apart and pack the bearings on all the suspension links, wheel bearings, and steering head. Things last a lot longer and you avoid break downs.

Serious racers do this. It is advocated by those at the bike forums.