If it isn't flat enough to 'resister' against the chip breaker, you will know it.Unlike a chisel, the back of the plane iron doesn’t act as a registration face.
If it has flatness issues at the edge it will have trouble with taking a light shaving.
Many of my old blades were not flat at the edges and had to be flattened in order to take a full width shaving. This led me to post on using a light back bevel at the sides of the blade to camber a plane iron.
Here is a shaving from the blade after a fresh sharpening:
Full Width Shaving.jpg
Here is a shaving from the same blade after a few rubs of the back corners of the blade on a hard Arkansas stone:
%22Cambered%22 Blade Shaving.jpg
It only takes three or fours strokes to get change the edge profile enough to change the nature of a thin shaving.
jtk