Yes, it is advisable to own a car or at least have access to a car in Nicaragua. Whether or not you need a car in Nicaragua also depends on how much traveling you expect to do.
If you are living in a small town, cabs, whether they be the little pedicabs or the "tuktuk" cabs as they call it here, or the horse driven cabs are all inexpensive everywhere. It is easy to negotiate with the drivers and that applies to every town and every major city in Nicaragua.
If you live in Managua, you are going to need a car and a driver but you can get away without having a car in any other small town in Nicaragua. If you want to go away for the weekend or drive to Managua from your small town like Granada, for example, sometimes it is even more cost effective to hire a car with a driver for US $50 to $75 per day. You can spend the whole day doing anything you want to do, go to all the shops you want to go to in Managua and come back to your town. You will not have to worry about parking, gas, or insurance.
Some people feel like their freedom is taken away if they do not have cars. A car can be something that some people might find hard to live without but I find that many people who bought cars here in Nicaragua never drive them. They drive them once a week or once a month just to go to Managua. They only bought the car for the sense of freedom. They can get their car and hit the road any time they want which is an American tradition and it is sometimes hard to break traditions.
You can have all kinds of cars here but you do not need them. It is an American tradition to have a couple of cars that serve different purposes. One car is for the weekend, and one truck for going back and forth to work. You can do that here in Nicaragua, too but you do not have to.
I personally cannot live without my truck but that is only because I am not retired. I go around a lot and work in three cities at the same time. I work in Granada, I have a farm in a little town called Diriomo (45 minutes to 1 hour away from Managua) and everything I need for supplies is in Managua. I am in one of those three towns every day and this is why I cannot do without an automobile, but most people could.