Yes we are sure.  Very sure.  Especially for a system which requires frequent additions of water due to evaporation or "boil off".  We call added water like this "make-up".  A good rule to follow:  if you are adding 5% or more of your total system volume each month due to evaporative loss (steam) you are heading for trouble if you do not improve the quality of your "make-up" water, ie soften, etc.  Mineral scale deposits will be the result.   NOT a good situation.  Decreased efficiency.  Drastically accelerated corrosion under the deposits.  Costly clean-up and repair.  A "closed" pressurized system would be the exception to the necessity of soft water feed because of the little to no make-up required.  The reason why softened water mistakenly comes under fire is because of Chlorides.  High Chloride concentration is definately to be avoided and is very determental to your system.  The bad rap for softened water comes from softeners that are not functioning properly, and as a result, some of the wash brine (HIGH Chlorides) is fed to the system.  So when you fill make sure your softener is working properly and make sure that the resin capacity has not been exhausted (ie softener can't keep up with demand).  Regenerate just prior to filling and know when the softener will be "out of soft water", needing regeneration again.  Flushing some water following regeneration would be good practice.  Some softeners don't have the capacity to fill an entire boiler volume on one cycle.  Our Soft Water Solution does not regenerate with the use of salt so therefore poses no problem with Chlorides being accidentally fed to the system.