That's always been my approach to building too Beetaloo.
Say the span tables over a 4m doorway calls up 2 x 250mm F11's nailed together as the minimum - then we'd allow for 2 x 300mm F11's . The number of times I've seen windows and doors binding because of sag in the lintels die to someone cheaping out and using the minimum. Same as for tie-downs, bracing etc etc.
Stewie