I suppose it depends upon the causes of the stress.
Whilst it is nice to have someone to talk to about your stresses, how you cope with the stress and live your life is more reliant on your personality than the relationship you are or are not in, and the people around you generally.
In my previous long term relationship I led a very stressful life with my career and the relationship was not able to sustain that. In my present relationship I lead a stress free happy and contented life but this is more to do with my own career circumstances and how I have learnt to deal with stressful situations. People who knew me then and know me now say that these days I am so laid back I am horizontal, but that is owing to the years between the two relationships as well being within them and my own emergent personality (can�t let the wife take all the credit).