Plastigauge is great little product from this firm:
it's a soft, pliable material used for checking actual radial running clearances between shaft and housing. The material is non-invasive, ie: if you get any in the engine it doesn't do any damage at all. I rarely use it unless I've had a crank that has run a bearing badly, relying more often than not with just micrometer measurements of crank journals and my experienced judgement of the feel of a crank in situ when it's torqued up and turned over.
To make sense of what the gauge tells you, you need to know the OE spec for clearance between the rod and crankpin or main journal and housings. For example:
In the case of the Fiat 16v Coupe block featured the main journal housing OE spec is: 56.717-56.730mm. The bore gauge measured typically 56.74mm vert (in line with bore axis) and 56.77-78mm on diagonal.
I loosened and shifted the cap from one side to another (they have a minute clearance when in good condition) and the diagonal measurement moved accordingly. So one reading in the diagonal plane should not be taken to mean the thing has gone way_out_of_round. On older blocks you get a fair bit of shift sideways between the main bearing cap and its locating slot in the block, and building up needs care. I like to tighten the bolts just over finger-tight and rotate the crank to seat them on centreline before fully (and progressively) tightening them up.
I have NEVER resorted to line boring a TC block!
See sequence below: