Basically the transition form a Swedish massage to a deep tissue massage is just a matter of degree.  There is good reason to belief that massage helps with body aches and increases circulation where circulation is needed to heal muscles.  However there does not seem to be conclusive evidence about the therapeutical effects.  Common sense tells us that if massage is too hard and too fast it may do more damage than good.  When I go deep I take care to do it very slowly so that the muscles have a chance to adapt to the deep strokes.  You simply tell if you like the massage harder or softer.