Mulch any deeper than 4″ is excessive.  Also, make sure that you never pile mulch up against the bark or stem of your plants.  Mulch or dirt piled against the stem and bark of plants slowly strangles the plant.  When you mulch, the last thing you do I go around to each plant and brush the mulch back until you can thee the stem where it enters the soil and leave it that way so the plant can breathe…more on that HERE.