I know everyone swears by the 4 foot level for taping, but I switched to a 6 foot one about a year ago and my seams are way better. | Drywall Installers - Topic Shares
Ever try an 8 foot level? It sounds crazy but on a long wall it's the same idea pushed further. You're not just fixing the seam, you're making the whole wall section flat between the studs. A short level can still let a hump hide between your tool and the next stud. The long one bridges that gap and shows you the real problem.
Switching to a 6 foot level makes sense. The extra length catches more of the wall's curve, so you're not just flattening a small spot. It forces the seam into a truer plane.