If your staying F2p then you don't need any prayer in my opinion.
For example (this figures are just approx values):
50 combat, 65 str - You hit 16's with strength pot etc
50 combat, 60 str, 31 prayer - You hit 16's with strength pot, prayer etc.
Those figures aren't exact, i'm just pointing out that prayer does absolutely nothing at all in F2P. 31 Prayer runs out so quickly that its not worth it anyway. In P2P at least you can drink prayer potions, but thats not possible in f2p.