Yes, you do for the refrigerator purpose if nothing else.

You will hear people say the use anything from a can of beer(or soda) laying on the floor testing it front to back and side to side, if it doesn’t roll, I’m good to using a cell phone app to use an advanced level.

I got down the advanced level route since the might vary slightly how many degree off level you can be and operate the fridge “safely”.  Safely being the important word here, take for example, the information supplied to e in class while learning troubleshooting on a Norcold refrigerator:

Today, Norcold recommends that their refrigerators operate within 3 degrees off level side-to-side and 6 degrees off level front-to-back.

So the question then is HOW do I tell the degrees, some people say half a bubble on a level, you will hear all kinds of things.

Here is what I use and it tells me EXACTLY the degrees off: