Agreed with LHR/MEL/Europe FF post.

Find out in advance if they have access, if they want it, and if not then ask if you would like to arrange something and act accordingly. No need to be a sycophant, no need to cause offence, be helpful.

I often fly a lower class than my contractors, if they want to pay extra for the seat, it is their choice, but there is no doubt there are some people whom are overly sensitive about hierarchy. However when flying for clients I strictly follow their corporate travel policies.