There is no formal convention for including people on a cc: list.  Some
of us do it, some don't.

My view is that many of us read the lists by means other than subscription
to the list.  Those other means can sometimes have downtime, or may even
lose a message.  (Or we may simply fail to notice a reply if the list is
particularly active.)

Unless the person says "Please do not cc: me," I normally include them.
And I *always* prefer to be included, myself, if the reply has any
bearing on something that I've said on the list.  I'm sure you'll hear
from others with opposite views.  (^&

-- 
  "I probably don't know what I'm talking about."  http://www.olib.org/~rkr/