You wont see a 2 thou bend in a lathe bed extrusion, but it will make a 4 thou on diameter change. Getting them to be straight and twist free is the real answer for setting up any lathe bed. Leveling a machine tool is the term used to get it to be as true as it was when made in the factory and working perfectly, before it was packaged and shipped. It does not mean that the spirit level is level everywhere. But if it was, it would also be good.
So there are things you can do, like make things that clamp it to get the twist out of it or to twist it to get it true. You may need center blocks to push it forward or back to get it dead straight. Its a topic in itself setting up the bed correct. Then the head stock needs to be correctly aligned to the newly leveled bed. Lastly the tail stocks need to be centered and so do the steady rests. The fixed ones are what I start with, and then adjust all the adjustable ones.
A taper bar for the most part negates the importance of the leveling of the bed, but it will be different in it's alignment where ever the centre is repositioned to.
Neil