The ruling on supplicating for death for one who fears harm for himself

Is supplicating for death when one fears harm for oneself recommended at all times, or only when one fears fitnah (trial, discord or temptation)?
One supplicates: "O Allah, by Your knowledge of the unseen and Your power over creation, keep me alive as long as You know life to be better for me, and take my soul when death is better for me." If he says this... or says: "O Allah, keep me alive if life is better for me"; there is nothing wrong with it. The Prophet (peace and blessings be upon him) granted a dispensation for this and did it himself; he granted a dispensation for it and did it.