I disagree slightly with the fact that immediates won't fit so well. At the beginning they should be perfect if the impressions are taken properly but they do lose their grip as your gums start shrinking but then you get reliners periodically until the time comes for you to have your permanents. Even so, I much preferred to have immediates and not be without teeth, even if they hadn't have fitted so well.
Make sure that your dentist gives you a total price. My dentist gave me the price of dentures which was fully inclusive of 2 reliners as and when needed. In the event, I never needed any adjustment at all to the top denture and over a period of 9 months just 3 reliners to the bottom. Normally you don't have to wait as long as 9 months for the perms. but I left it that long to make doubly sure my gums had got to where they were going and also because of timing and financial issues.