Hi Nick,
It does not really matter as long as it is rigid enough for what you want.
Machine up for accuracy as required...or leave as it is if the angles are right. You should machine the base at least so that there is no stress or flex when mounting to a flat surface, and you don't damage whatever you mount it on to.
I have got several bits of hot rolled angle (you can tell on the inside of the angle; it is not square but looks like cast) and I have no problem with drilling/grinding/cutting/whatever as they cost nothing.
I do have a couple of proper ones too but I am much more gentle with those!
Andy