Many states have no smoking bans in restaurants. This eliminates the need to give a smoking preference when being seated. However, in states where there is not a smoking ban, seating preference can become tricky when entertaining guests.
If you are entertaining clients and do not smoke, you are not obligated to sit in the smoking section. If your client asks if you would mind being seated in the smoking section, be honest and explain your sensitivity to smoke and tell them you would appreciate his or her understanding to sit in the non-smoking section.
When your client agrees, which they should do, thank them for their understanding.