I have very rarely seen a standing breech that was in line with the centre of a barrel, once fit to the hook. The tang frequently needs bending, not only to conform to the breech/wrist area in profile, but often left or right of the centre line of the rifle. If you don't inlet it as a unit, you will have a difficult to impossible time of getting the faces of the standing breech and the breech plug to align again.
I solder mine together so I can apply considerable bending force to get the tang down the centre of the stock...especially true in Hawken style hooked breeches and tangs. Heat separates them when the job is done, and scrapers remove the solder easily. Cold blue tells me where there is solder that still needs to be scraped away.
The fit of the breech plug to the standing breech in a hooked breech firearm is crucial to accuracy, so it has to be right. the bedding of the standing breech in the wood too, it extremely important. A perfect job is the only one acceptable, and fitting the tang and barrel as a unit, once the barrel is fit to the channel, is the only way I can do it correctly.