Under the kick panels is always nice....if there's enough space for them. If not, you can always put them in the trunk, under the front seats or anywhere they'll stay dry.
I wouldn't put them in the trunk. Longer cable = more resistance. I'd keep it short and make a nice display out of it. Bumpin' Goalie has a nice idea there
In my wife's Altima, they are in the dashboard...under the glove box, and under the steering column. In my Maxima, they are both in the center console, under the dash, beneath the radio. I'm not a fan of putting them in th doors.