It definitely depends on the tire and if the cracks are superficial or structurally detrimental. I generally ride most tires; even questionable ones (though, not on far treks). Gumwalls like you've got there definitely don't hold up as well as a blackwall. They tend to dry up and look real nasty before their time but the rubber underneath may still be strong and rideable. Examine it up close, also on the inside of the tire for splits, and use your own judgement.