Jason,
The requirement of this Preheating technique is
dependent on the level of quality desired.  I have
instructed many companies throughout the world and
have received many questions regarding this process.
The concern is that of thermal expansion within the
capacitor.  This too sudden expansion can cause the
component to crack internally and thus is not visually
detectable.  This crack may also not be sufficient at time
of installation to be be located as a fault during testing.
But with thermal cycles from normal operation, this crack
may propagate into a failure.  This possibility of cracking
while in the field is what should determine the need for
installation utilizing preheat.  It can happen.  And if so, is
this acceptable?
Many companies do not feel this preheating is necessary.
But I must report I have had contact with customers
having difficulties with capacitor failure and once the
preheating process is established, for some reason, the
failure rates greatly decrease.  And I know you will
receive feedback many companies do not preheat and
have no problems at all.
This preheating procedure was brought to the industry
attention many years ago through various articles and
evaluation reports from component manufactures,
consultants and industry periodicals.  While the
information within this literature is still valid, it should be
noted the component manufacturing today has greatly
improved from when this component cracking started.
But still today the component manufactures normally
state the manual soldering operation should duplicate the
manufacturing process.  In doing this, the manufacture
provides a ramp rate which is recommended for proper
installation.
You also stated there seems to be no problem if the
"operators take care".  If you have operators who
ALWAYS take care in every installation, you should be
commended.  But if one does not, how can this be
determined and is the lack of the preheating process
worth a possible a field failure?

Marshall Canaday
Applications Engineer / Training Specialist
Pace Inc.
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