Hi John, getting the lathe perfectly level is not critical, but bolting it to the stand is! If you have a lathe which has a seperate plinth at either end, then the concrete it sits on must be flat and level, or the bed will twist over time. If the lathe is on a cabinet stand, levelling is not critical, but when you bolt the lathe to the cabinet, you can twist the bed. I would assemble the lathe loose onto the cabinet, then do some measuring with a test bar, or checking for taper turning by your favourite method, taking a cut off two collars say 6" to 9" apart is mine, then gently bolt the bed down to the stand and check again. To level the stand, just do as Andrew suggests, but you may have to adjust it away from level to get good coolant drainage if you are using coolant. I have two lathes, a Cochester Student on a cabinet stand, and a 13" Covmac, which has a supporting plinth under the headstock, and two cast legs with feet (actually all one casting) at the tailstock end. If this went on a floor that was not level, even if not bolted down, you would expect the bed to twist over time, unless you put shims under it to even out the support. If your new lathe has a sturdy cabinet, I would not bother fixing it down at all, just use levelling feet, or shim it till it is firm..
Phil