My experience is, if you can place the plane on a known flat surface such as a tested table saw top or a planer bed, have no rock n the plane, can see no backlight anywhere under the sole and/or can’t work a thin feeler gage blade in anywhere along the sole, you are flat enough to do the work, especially when using camber on the blade. Don’t worry about small gaps along the front, back, or side edges since those areas are often eased to facilitated planning anyway. Again, in my opinion, super tuning is best left to smooth planes and some joinery planes and is wasted effort on planes used for general stock removal, especially scrub and fore plane type work. Squaring the sides should be reserved for tools used for shooting and squaring operations where the sides help control the cut. Even with that, some out-of-square-ness can be compensated for through lateral adjustment of the blade.