Crit Chance runes round!
Just tried out the practice tool with 2.5 attack speed. If you have 0.28% crit chance (1 blue), you won't ever crit. But if you have 0.56% crit chance (2 blue), you'll crit once in a while. After 4 minutes of continuously attacking a dummy: 1 blue gave 0 crits, 2 blue gave 6 crits. A second trial of 6 minutes of continuously attacking a dummy: 1 blue gave 0 crits, 2 blues gave 5 crits.
So crit chance runes get rounded...