That will depend on the product, how many upgrades you have, your supplies quality, and the current market situation.
Each type of product the percentage of total salary cost will vary. If the cost of salaries is lower you can raise salary.
If you have higher upgrades, higher supplies quality, and if the market price is higher than base prices then it probably makes sense to raise the salaries.
Most people use one salary level because it is much easier and we is lazy.
The other concern, especially for CEOs is competition with other corps and a country's military. If your salaries are low you might not have workers.
For your state corps don't forget that the workers are paying more than half of the salary back to you as taxes.