Spacers or offset wheels are the same as far as stress on the bearings goes. The benefit of doing it with wheels is you don't have an extra set of nuts to mess with. The benefit of doing it with spacers is you can take em off if you don't like them or use them with different wheels, or try different sizes. It's a good idea to lift each wheel off the ground and give it a wobble on a regular basis and that's more important if you're running spacers / offset wheels / big tyres as there will be a smaller margin for error. There should be no detectable play in the bearings, not even a tiny bit.