I think the answer is to level it at what ever is most important for you to be level.
You'll get plenty of different answers. Some people will level inside the fridge or freezer thinking that they have to be perfectly level while operating - I believe that they aren't that sensitive. Others level to the counters, others the floor. In the end, all of it is probably close enough, just hit what makes the most sense to you.
Personally, I just hit the auto-level button and let the trailer decide what it wants to be level to.
The best answer I ever saw was a gentleman that leveled the rig to the cook top because his wife didn't like the eggs sliding to the side of the pan. That is one smart fellow!!