An automotive boring bar should stay straight.
On a lathe you can bore a taper, a few ways.
But in fitting a piston, the boring is the first step, then you finish hone.. You can create a small taper when honing...either on purpose, or by accident if you are sloppy.
Forged pistons are usually set up looser, they have different expansion rates.
The only reason to set up anything loose, it to be able to race without breakin...or maybe because you cant measure accurately.
