While I agree that the flatness of the sole is not likely to be an ultimate issue, I am of the opinion that, the flatter the sole around the mouth area, and along with a coplanar toe/heel, the thinner the shavings can be. This is important when - naturally - just wanting to take the finest, thinnest shavings when either fine tuning or finish planing.
I find it interesting that I can take the finest, even shavings with a Veritas BU Smoother (62 degree cutting angle), and other BU smoothers, more reliably than with a Veritas Custom #4 smoother, and all other BD smoothers, with a closed up chipbreaker. Note that I probably use the BD planes a whole lot more, for almost everything, but when the finest shaving needs to be taken, and the money is on the table, I am more likely to pull out the BU smoother.
I have never placed a straight edge on a new plane's sole.
Regards from Perth
Derek