I have driven less than 2/3 of the mileage proposed by my Nissan dealer for when I needed regular maintenance. However 6 months have passed since my last regular maintenance, and I've been getting notices from the Nissan dealer to come in.. Car runs great - should I take it in for maintenance now, because 6 months have gone by, or wait until I've driven the full mileage, which could be another 3 or 4 months?

 

You may find you are able to save some money by using a reputable independent shop but the easy answer is-
If you're flush, have it serviced at whichever comes 1st, miles or calendar, it won't hurt if done correctly. If you need to be thrifty by all means go by mileage (assuming the car is driven at least weekly). And keep all your service records as long as you own the car.