Answer:
I guess it all depends on the situation and why you are praying this. God sees the heart and knows the motives.
I prayed for my own grandfather years ago when he was in a lot of pain from cancer. I prayed that God would cut short his days of suffering and take him home to be with Jesus. He had lived a full and good life, and loved God very much.
In my mind it was not praying for my grandfather to die but to live forever with Christ.