How common are worms in dried mushrooms?
It must be pretty widespread (at least in morels) as evinced from this pretty extensive list of references to worms in fresh (and dried) morels.  From what I've read, the presence of worms seems to indicate that the mushrooms are of lower quality and might have been harvested too late.


  Dave Fischer's North American Mushroom Basics - Some fungi have
  evolved to take advantage of multiple food sources. For example, the
  Oyster Mushrooms you can buy fresh at many grocery stores break down
  and digest cellulose, but they have also developed mechanisms for
  literally trapping and then eating tiny little "worms" called
  nematodes; this gives them access to extra nitrogen...".
  
  Mycological Society of San Francisco - Avoid morels whose caps are
  soft or mushy, or become granular when rubbed: they are too old and
  wormy. Morels occasionally contain insect larvae that drop out during
  the drying process. The mushroom-lovers we know have disregarded this
  aspect of morel enjoyment. After all, they are very small worms.
  Because of the irregular nature of its surface a morel cannot be
  rubbed or brushed. You may find this worrisome, wondering about what
  kind of things lurk in the dark pits ready to jump into your b'chamel
  sauce... 
  
  Sandy’s April Product Report - When purchasing morels, check carefully
  for worms and excess sand. The worms should be easy to spot as they
  are white, in contrast to the dark mushroom. To clean, gently brush
  off any sand or dirt with a soft pastry brush.