Extremely arguable topic. Everyone has their different opinions on it, so it's hard to say what stats you should get up. Overall, getting a few extra combat levels doesn't really do any harm.
If your a low level, it doesn't really help. When you get around 40 attack, 75-80+ strength, then i advise you to get 31 prayer. If your not used to pking with prayer then don't do it, but if your used to turning something to do with prayer on during a fight i say get 31 prayer.