I believe the five tigers were the strongest generals of shu at that point. They were the most feared generals througout the land at that time. Most of Wei Yan's actions in battle were after appointing the five tiger generals. and after the death of three of them. Lets not forget that Liu Zhang surrendered simply because Ma Chao was heading towards Cheng Du. I think Ma Chao's appointment as a five tiger general is just, he showed his skill and valor on enough occasions to earn the title.
All we need to decide is what to do with the time that is given to us... "Gandalf the grey"
Death smiles to us all...., all we can do is smile back... "Marcus Aurelius"
Freedom lies behind the boundries we've created ourselves...