Can't help with the insulin side of the question as I am only on Metformin, what i can say is that normally I am advised not to eat or drink for 2 - 3 hours after having any fillings done, and when I had an extraction a few months ago I wasn't to eat anything for several hours and then only soft food for a day or two.
Some fillings need time to cure so it might depend on which type you have as to whether you can eat straight afterwards or not, and with extractions you need to do your best to make sure the blood clot sets well and doesn't break otherwise you end up with a dry socket which is not only exceedingly painful but takes longer to heal and normally needs extra treatment by the dentist to pack the socket. I should mention that I am terrified of dental treatment and for the extraction need to be sedated as well as the normal local anaetheic so I do try and do what ever possible to minimise the follow on treatment.
I would say therefore, you are best to eat before the appointment and plan not to have anything else until about lunchtime.