I used to be fanatic about it, due to the fridge. Then I realized I owned a digital level, so I took it along and measured the tilt. As ksg5000 wrote, if you are comfortable, the fridge is fine. It takes a lot of tilt to get to the point where the fridge can be affected/damaged.
So, we rarely use anything. But if we have to, we use the orange leveling blocks we bought in the RV section at Wal-Mart. They take up less space and weight than lumber does. I used to have lumber for the job. We don't have enough storage for me to haul it in this current C.
It often happens that we have to park quite a bit off level while at a store or something. In that case I turn the fridge off. I have a plastic spring clothespin at the fridge, and I clamp it on the gear shift lever to remind me to turn the fridge back on before we get moving again.