Most of us are amateur builders, who make longrifles for fun. So, who is to deny you the fun or taking every step possible to make a rifle or smoothbore as nice as we can? Go ahead and inlet the whole thing, and avoid taking shortcuts when possible.
There are others here on the forum who make products for sale. Profits being marginal, at best, they need to take reasonable steps to decrease the time necessary to make a very functional, very beautiful product. And it sure does no harm to only bed-in the first and last 6" sections.
Me? I'd rather bed the whole barrel - prolong the fun, so to speak.