How important is proof of freelance work for a TN visa?

Hi everyone. I have a couple questions regarding applying for a TN visa.

I have a 3 year diploma for graphic design and also three years work experience in that area. While some of that comes from working at a design firm, most of that comes from freelance work. How important is it to prove to an immigration officer that I have done freelance work?

How would I go about doing that? Do I show up to the interview with my portfolio? Do people with largely freelance work histories tend to get turned down a lot?

Any information or past experiences people have had with this kind of thing would be great!

Can't see why it would be relevant, plenty of doctors and the like go in who have been essentially self-employed up to that point.