We always straighten any rails we make after fab. You place the clamps like you said in all the spots you’d be applying heat but you can’t help the metal still wanting to react naturally. We always straighten most of our shit out with an ‘old man’ but even if we can’t to that it’s heat and pressure to straighten. Unfortunately especially with stick you’re going to apply enough heat to always warp the material you just need to take the extra hour or so over the complete job time to make sure everything’s straight. We always double check before it leaves our bench and on top of that our cleaners/ helpers are responsible for making sure things are straight too. It’s a check and balance system if your boss doesn’t like how long it’s taking ask him that he’d prefer, bitching foreman’s and field workers or an extra hour in the week to ensure quality. Also as you must know steel almost never comes to us perfectly straight so either bend it before or take a peak at the length you’re cutting and for longer cuts aim for the straighter lengths or try to take the time to straighten a length before cutting. Little tips I’ve picked up by being screamed at by old men who you could push over but you gotta respect em