Re: blueprinting 700 action
Replacing the bolt tightens tolerances between the bolt body and bolt raceway. Factory tolerance runs about .004-.007. After reaming the raceway and replacing the bolt, you can choose your tolerance, .002 is common. The tighter tolerance holds the bolt face closer to proper alignment before firing. However, upon firing the lugs will be forced, under high pressure into full contact, properly aligning the bolt face.
The upgrade is entirely optional. For a hunting rifle, I wouldn't bother unless you were planning to flute the bolt and add an extractor. In that case, the new bolt doesn't cost any extra. For a target rifle, I would do it.