As far as i understand posers strong point is pose to pose animation, where every pose (extreme) being a key frame, I dont think it will make things easier if you add key frames every 15 frame or so, just to do it as you risk getting "chunky" or messy animations. But instead add them where it makes sense.

To explain it a bit easier with a simple example, we could imagine having a ball bouncing across the screen. 

If we say that the time it takes for the ball to do this action is 2 seconds we would need 60 frames at 30 FPS.

We start out by adding 3 key frames.

First key frame is where the ball is in its starting position in the air at frame 1. To keep it simple we say that the ball will hit the ground after one second, so add the second key frame in frame 30, and move the ball to the ground. And last move the ball to its end position at frame 60.

If you play the animation now, it will look rather weird as the ball moves in straight lines between your key frames. To fix that you go to frame 15 and frame 45 and adjust the ball so it moves more like its bouncing.

And its basically the same with characters, there is just a lot more to take into account. So most important you should know or estimate before hand how long you assume that the specific action should take, and then add key frames in the extreme positions, where you actually need them and then you can always adjust what happens inbetween, to fix up the animations.