For successful trading it is important not to waste units! Thus you should check before a battle how important it is. Land Battles can be divided into two categories: Countries that have to be taken in order to prevent that the enemy can fortify these by using his aircraft and countries that only increase your income. The first should be attacked with 2 inf + air if only 1 inf is present, with 2 inf + many air if 2 inf are present or 3 inf + air and so on?.
But in case of bad luck you must be willing to sacrifice an air unit.
Example: Ger has to conquer Eeu with 2 inf stationed there. Ger has 4ftr and 1bmb available as air support. Therefore it is recommended to attack with 2inf + air, because this would lead only to a 1/8 chance to sacrifice an air unit. 1/8 of a fighter costs 1.5 IPC ? in contrary an additional inf would cost 3 IPC. In that situation you would save 1.5 IPC on average.
But if less air support would be available the third inf would certainly make sense.
Another example: Ukr is defended by 1 inf and the attack is carried out only for an income increase. Again 4ftr + 1 bmb are available for air support. If you use 1inf, you have a 66% chance to take the land, this means 66% for a results that costs 3 IPC but also yields 3 IPC (and 1 IPC, because the allies have a 33% chance to be hit at the time they try to recapture the land). Altogether you have a 66% chance to yield a profit of 3 IPC (Rus-inf (3 IPC) + Ukr-value (3 IPC) ? Ger-inf (3 IPC)) and a 34% chance for a draw (exchange of inf-units), which means that you have an average profit of 2 IPC (66% * 3 + 34% * 0).
But if you would attack with 2 inf the result would change: There would never be a chance to yield anything overall. Certainly the used calculation is not completely true because the opponent would also need additional units for the attack.
The general rule is: Trading attacks that are only used to increase your income should be accomplished using the following guideline: 1 inf + air for 1 inf, 2 inf + air for 2 inf and for 3 inf you should use 3 inf + air or even only 2inf, if many air units are available. In long term this strategy will pay off and will give you an advantage over an opponent who does not follow this guideline.