Player Piano Company in their catalog recommends the best crease in
bellows is _no crease_ at all:

  "If the cotton pneumatic cloth is never creased, but instead allowed
  to fold naturally by vacuum under a situation where the pneumatic is
  prevented from closing by 1/8", the wear life will be increased by
  three times.

  Most player manufacturers placed a felt punching inside the
  pneumatic, just behind the fold, to prevent the hard crease.  A few
  makers did not do this, and the result was pinholes and slits on the
  folds after about ten year's use."

Any other opinions or experiences about this?

Larry Lobel