as far as blowing out air filters, it depends greatly on the operation* and it is a MUST to check the filter for tears and small rips after blowing it out.
*= in a brick manufacturing or other operation that is very dusty, (paper recyler, concrete plant, etc...) on that is requiring the blowing out of radiators on a daily basis, then I would blow out the air filter at least weekly, and check it by shining a flashlight from the inside and if I see light, it gets replaced, and it still gets replaced monthly.
you can buy a lot of air filters for the price on an engine that has seized a bearing and thrown a rod through the block.