One way to check to see if the rods are flexing due to weight would be to remove the weight, by removing the extruder assembly when testing.
You’ll find though, that any variences work themselves out rather well. Even if there was a difference of 0.5mm across the bed/printing plane (for whatever reason), with the multiple solid bottom surface layers it would work itself out. The first layer may be thick on one side, and thin on the other, or towards the middle, but that would be both covered and filled in by the second and third layer.
Sure there may be a theoretical difference in the height of the model after it’s been printed, a difference of 0.1mm - 0.5mm is negligable at the scales we are printing- even (12^3). I’m sure that some of the beds in our fleet of 3D printers have beds that are not perfectly level, even with that, we are able to crank out beautiful well-fitting parts after part.
In my personal experience with several different 3D printers, both my own and my friends (Prusa i1, i2, i3; Printrbot, Replicator, Rostock, TAZ, TAZmini, AO-101, Cupcake, Tantilus, MMax 1.5, MMax 2.0, etc) we have yet to come across any issues with a printing plane that’s not perfectly flat. At the scales we print at, (0.1mm - 0.5mm layer height) sub 12^3 inch items won’t show any kind of issues.
When printing at sub 0.1mm layer heights, you’ll typically be printing something small in scale anyways, or encounter other issues before the printing plane causes any hiccups- due to how well your slicing settings need to be balanced.