what can I say, sure, level your lathe, go for it. I'm just saying I've never see noticeable taper in either of my lathes (Atlas 618 and South Bend heavy 9) over 6-7". If you have a problem, fix it. If you don't, don't (unless you want to).
I think the issue here is that "level" is a bit of a misnomer.
What needs to be fixed/adjusted/confirmed is that the bed ways are true and parallel to each other and the headstock, tailstock and carriage are in line with the ways.
As long as those parameters are good, you could stick the lathe to the ceiling upside down and it will still cut true.
We use a machinist level on the lathe to get the ways true to each other because it's a relatively simple and easy way to do it. The bubble gives you a common reference plane to make your adjustments to. Once the ways are both level in the length and width to the machinist level, they're true to each other.
If a guy's lathe is cutting true without doing any shimming or alignment, then the ways are true to each other and the headstock, tailstock and carriage are in alignment.
Chucking a machinists level on it at that point just confirms what you already know.
But herein also lies the "gotcha" of not having a solid, stable, secure mounting and why guys get the "loose the casters" comments. It's also why wood isn't the best mounting surface (can warp, twist and shrink expand with heat and humidity changes).
If the table/cabinet can move/flex, it can cause the bed ways to twist (we think of cast iron as stiff, but it's not immune to movement and shift) which throws it all out and you start getting strange things like tapering, inaccuracy, not repeatable, etc. This is where chucking a machinst's level on it can be useful if you move it around. You can confirm it's all good before turning something and wasting your good stock material.
