My company has well-defined rules on what booking classes we can use for travel, so there's no wiggle room. However, they do make allowances for employees who have to book refundable tickets because of the need for flexibility (and it's simply cheaper to have a refundable ticket than to deal with the change fees). I've also gotten upfront approval to purchase a higher fare class for the purposes of upgrading from my manager (and, his manager) when budgets were cut and we couldn't book business class any more.

Booking a more expensive ticket than necessary for personal reasons does seem to be shaky ground. I would err on the side of avoiding any appearance of impropriety and ask Finance to clarify the rules.