Weight equals sag . So no matter how well you grind or mill it with support it can sag without support . So rigid bracing design but lighter weight will give far less sag once erected .
If you have support options along the length then most sag can be shimmed out . However once you have support along the length you don't need excessive strength .
Welded bracing design can warp and your back at stage one. It all comes back to how the beam is supported as to which beam cross section and strength is best .
A beam that is reasonably straight can be clamped down shimmed and bolted , made straight using a good straight edge and an engineers level . High spots can be found by moving the straight edge from side to side and see where it pivots . Low spots can be found by placing paper strip under the straight edge and see where it pivots or just pulls out . Engineers level can track along the whole length turning the level each time to see if it's straight and level or if there's still humps and hollows.