Hmm, this is interesting. I never change the power by holding the direction. Basically, what I do is simultaneously tap shoot + left (panenka) or right (hard shot) and only during the run up do I hold up/down to aim for the posts.
It seems to work fine, but I guess in my next penalty I'll see if I can change the power of a panenka. I seem to remember seeing a video of a ridiculously soft panenka kick that hit the post and the gk didn't react at all even though he didn't dive....