Do I need to check isDisposed() before calling dispose()?

I don´t think so, if you check i.e. implementation of CompositeDisposable(of course if you using that class, not another implementation of disposable interface) there is :

 @Override
    public void dispose() {
        if (disposed) {
            return;
        }
...rest of method body
@Override
    public boolean isDisposed() {
        return disposed;
    }

So the answer is it depends in what way Disposable interface is implemented, You can check it in JetBrains IDE by right click and go to> implementation

It makes little sense to call isDisposed. dispose implementations already do that for you and make sure repeated calls are no-ops or have no detectable effect.

Unfortunately, somebody in the early days of RxJava started writing examples with it and now everybody keeps copying that pattern.

It makes a little more sense to check isDisposed before calling onNext for example but you don't get to do that very often outside Observable.create().