You are fine playing just one job, but my advise is that you should at least watch a video guide on other jobs to know what they are capable of and avoid situations where your AST drops Earthly Star and you step out of it, you pull a group of mobs outside of NIN's Doton, or your WHM used Benediction a minute ago and you are about to die and don't use Hallowed Ground waiting him to Bene you.