Tires Inside/outside ??
 

What is the reasoning for inside / outside tires, for like all season.
I'm thinking it's not so much for the tread pattern because when you do that the pattern is going to be reversed on one side or the other.
Is it more for the sidewall??

How critical is it?

I'd ask a tire forum rather than these guys. You'll get so much contradictory info your head will spin.

Tires Inside/Outside
 

Generally, all season tires do not have an inside or outside. However, the full DOT (Department of Transportation) serial number (11 or 12 letters and numbers) should be positioned so that you do not have to crawl under the vehicle to read it in the event or a recall by the tire manufacturer or government. Some manufacturers sell white striped or white lettered tires with the opposite side (black sidewall side) decorative enough to be mounted with the black sidewall to the outside. That then puts the DOT serial number on the inside which is not optimal.

There are certain tires designed with what is called asymmetrical tread pattern and these do have inside and outside. Normally the "OUTSIDE" is designated on the tire's sidewall. There are also some tires that are designed so that the tire is supposed to rotate in a particular direction. The manufacturers will add wording or arrows to the sidewall to indicate tire rotation direction when moving forward. Tire manufacturers are supposed to take these 2 special cases into consideration so that the full DOT serial number is readable without having to crawl under the vehicle.

So in reality, what is important is to have the tire's DOT serial number on the outside so that it can be easily read in case of an tire recall.