There are two schools about this. When i was a dealer I would just drop a couple of new bearing in the housing. Trouble was I never got the life out of those spindles than if I would have gone with a complete assembly. When the factory installs the bearings there are put it properly and in line with each other. Field replacement isn't always done correctly and bearing life is shortened.
In my older wiser age, I just replace the whole assembly. It's quicker, easier and stronger.