I think that there are two main groups within the Witnesses as far as this belief is concerned.
There are those that don't fully believe it--whilst somehow convincing themselves and others that they do, so as not to appear weak--and are being financially responsible by making sure that they have a decent job, some savings and a pension plan.
On the other hand there are those that do just enough work to tide themselves over as they "work for Jehovah." I am acquainted with a newly married self-righteous pharisee and he is the epitome of the smug, overly-righteous type who truly believes and who has no plans for the future. In fact one of his circle rudely laughed in my face when I said that I was currently working towards a professional career with a good pension scheme. I wonder who will be laughing in forty years time. I try not to think such vindictive thoughts, as the JWs really deserve pity, but they truly do make it difficult for one to be charitable sometimes.
It is truly saddening when contemplating the future of the "true believers" who have been taught that financial irresponsibility is a virtue.