Hi there,
I think that some of the most intriguing paintings of people that I have seen either show perfect teeth, no teeth or exagerrated awful ones. So, if the teeth are perfect or if you are going for a portrait of a real "character" then say cheese. If not, then consider not doing it.
Kevin Aucoin, the famous makeup artist for models said to never say "cheese" when posing with a smile. He said that the word "thirteen" positioned the mouth much more pleasantly. My vote is with the NOT cheese, or make the teeth as perfect as possible without losing the likeness, or make them insignificant by not adding too much detail to them. Good luck as this will be a fun project for you.