You could go broke vaccinating for everything under the sun that a chicken could ever get-some diseases are just not all that common. I vaccinate for nothing, never have, have never encountered contagious disease here at all. Most backyard flock owners who practice reasonable biosecurity and good common sense really don't need to do so, IMO.
It's your call, of course, but just know that some vaccines will actually make the bird a carrier of the disease you are vaccinating for you and you'll always have to vaccinate for it for the rest of the time you own poultry. You'll have to do research on this. And some need boosters, too.
ETA: One misconception is that Mareks vaccine prevents a bird from contracting it. It only prevents the tumors associated with it from forming, as I understand it, doesn't completely prevent the disease.