I have to jump in on this one , all of the replies are very
interesting only because I don't , (for the most part)
disagree with anyone. ALL points seem to be valid.

I will usually run two tanks of fuel at a light load and low
rpm on the ground , then just fly it on the fat side for a few
flights then lean it for the best reliable running, (not tweaked for every
last rpm)from then on. Why am I telling you this? Only to show
that I do at least some break in, does it matter ? probably . But
in some cases maybe not .

Consider this, How many chainsaws,snowblowers,lawn mowers,
weedwackers,etc,etc,etc do you think get" broken-in"? And yet
these machines run for years.

If you got a brand new snowblower today , and then got
12 inches of snow tomorrow, would anyone reading this
use a shovel because the motor isn't "broken-in"yet ?

Fill the oil,fill the gas, wide open throttle, GO !

Just another point of view
Roby