I iron my fabric with the grain lines on the back after washing it before cutting it.
It is easier to spot a printing flaw or a slub or - very rarely - a hole - from the back.
I avoid ironing on the bias - I feel that distorts the fabric.
I usually start at the center and work out to the selvages. If the selvages have shrunk/puckered - I clip them so the piece lays flat. Sometimes those selvages shrink up a lot.
I have a "Big Board" ironing board topper - makes ironing the full width of the fabric much easier.