I think the "excessive buildup" that the dealer referenced is probably in
your wallet! That said, I'm fairly certain that the procedure they are
recommending will indeed accomplish the desired goal.
Seriously, if this is an engine oil flush, then you should not need it if
you have properly maintained the vehicle and changed the oil and filter
every 3K miles/3 months. I have used the "oil flush in a can" (basically a
quart of kerosene that gets added to the 4 quarts already in there) prior to
changing oil on a couple of vehicles I purchased with >100K miles and
unknown or poor maintenance histories. They did succeed in improving the
oil color so I presume they did reduced the number of deposits/sludge in the
oil pan (never dropped the pan to check). But, I'd hesitate to use this
type of additive with any frequency since it reduces the engine lubrication
while it is in the crankcase. I'd also recommend not letting it idle more
than 5 or so minutes (and do not drive it) while the flush is in the
crankcase.
If it is an engine coolant (engine block/radiator/core) flush you are
describing, then yes, this is desirable, but you can most certainly do this
yourself fairly easily. But, it would have been best done when the weather
was warmer last Fall unless you have a heated garage or live in a warm
climate.
Bob