If I was the boss, I wouldn't really mind being guested by someone lower down in the company management chain. I'm confident enough about my person not to be offended by a junior or direct report having more travel privileges (especially if that person's job requires frequent travel) as long as it is not costing the company more than what policy allows.

Personally, I would only volunteer to invite a superior in my company if our working relationship/rapport is at a level where we are friendly outside of work.