Originally Posted by
inkista
If you are shooting in low light, and you don't have flash-think wired into your head, P mode is actually a good choice, because the A/Av and S/Tv modes on the camera default to a fill-flash flash/ambient balance. That is, it's expected that your exposure settings will be very close to what ambient (all the light that isn't the flash) would need, and that you're just flicking out a low level of flash to "fill in" shadows. Works great in the daytime and good lighting conditions.
But in low light, the problem is that with the A mode, what ambient can require might be a very slow shutter speed, even if iso 1600 and f/2. And it may be too slow to handhold or for the action. And you've got power to burn on the flash at those settings. P takes the ambient light level into account, and makes the flash work more like a P&S flash (which is what most flash newbs have experience with off the bat): fill in good light; but flash as main illumination in lower light conditions.
Obviously, you really want to get to the point where you can shoot in M on the camera, and control your flash/ambient balance exactly the way you want it--maybe even take your flashes off-camera at some point--but that's not gonna happen by tomorrow, especially while event shooting, bouncing, and scrambling to get the shots as they arrive. So, P and iTTL, and learning to ride the flash exposure compensation are probably the easiest way to go.