To some extent regardless of how you feel about the "Big Guy Vs. the Little Guy" arguments these big organisations do have a legitimate trademark claim over the term and someone could use the username with the intention of misleading others.

It isn't reasonable to assume you'll be able to reserve every single trademarked username. So all you can do is have a policy and process in place to handle trademark complaints. You may also want to inform users at sigh-up that usernames subject to trademark complaints may be lost.

Then if the company wants to utilise their trademark as a legitimate representation of that trademark, then I'd likely have a process there also to verify that they are who they say they are.