I don't think an EIN is absolutely necessary unless/until you have to start filing 5500 forms.

I started my Solo fairly late so I still have <$250K of assets and am still just using my business EIN.  If/when I get over the $250 I will apply for an EIN and notify the broker accordingly.

I'm at the point of burn-out on this stuff that I just have to trust that my younger self knew what he was doing at the time. 🙂  I have a calendar reminder every year in January to check the account balance for 5500 filing.

Rick