afaik and imho... cracks are different from bukol so that the following items should be considered:

a. how long were the tires used? baka naman kasi bago, eh walang kwenta yan. pero kung matagal and malutong na, eh di dapat naman talagang palitan yan....
b. location of the crack... kung sa sidewall, medyo delikado... pero kung sa loob, ok lang yan. take bogeyman's opinion.... kung sa loob, check out the tread wear. kung malalim pa tread, it means, makapal pa goma, so that the crack is insignificant. pero kung mababa na tread, any crack like that can affect the performance of the tire.