Cracks in deck supports
Should I be concerned?


It's good to be concerned, as many decks are poorly built. Fortunately, most 'cracking' of support columns doesn't affect the integrity at all. 

This type of cracking is called 'checking' and happens as the wood slowly dries out. Code typically is accommodating of this...hence the 6x6 (as opposed to a 4x4). 

If you visit an old post-and-beam built factory or warehouse you'll note that many of the major posts have this kind of cracking. The buildings are still standing just fine 100+ years later, though. 

It's highly unlikely that the posts for your deck need any structural reinforcement. Seeing the photos will help confirm that.