I've sharpened mower blades for years and years.

An important part of the sharpening step is to keep the blade cool. Keep a container of water that you can stick half of the blade into. Grind the edge a little bit and then cool in the water and then grind the edge on the other end and cool it in water. If it ever gets hot enough to discolor, that portion has lost its temper and you might as well throw it in the scrap metal pile.

Take your time and grind the edge at the same angle as original on the same side. Don't get the idea to grind on both sides of the edge. Also, don't roll the edge off of the grinding wheel, rounding off the end. This does away with its ability to cut on the end. Not a big deal on a single blade deck, but if it is a two or three blade deck, it will then leave an uncut stripe between the blades.

Yes, the balance IS INDEED important, but this is easily taken care of. Drive a long, thin nail somewhere. Hang the blade by the center hole. If it does not hang on the nail visibly level, simply grind a little weight off of the heavy end until it hangs visibly level on the nail.

You can get a cone shaped balance tester wherever mower parts are sold and it only costs a few bucks. I bought one a few years ago and it works well, but I don't think that it accomplishes anything that you can't accomplish with the nail.

There is a point where you have to give up on a blade and buy new, when you do, especially for a tough old machine like the old Cub, buy GOOD, heavy blades. You typically can't get them at the ag stores like TSC or somewhere. The OE blade on most commercial mowers are made by Oregon. They cost more, but will last longer and take more resharpenings. You might try someplace like JacksSmallEngines.com, or other on line OPE suppliers.

Hope this helps.

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2001 SLK 320 six speed manual
2014 Porsche Cayenne six speed manual

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