The only reason you ever need to use the instep is if you're shooting outside the box. Anything inside you should take the placement advantage of the inside of the foot over the power advantage of the instep. You can still get good power with the inside of the foot and be almost perfect in accuracy.
I would say my two best shots are bending the ball to the far post upper corner and the Stevie Gerrard bullet (which goes over a lot). For the bend into the top corner of the far post hit it somewhere between the side and the instep, get good power, but most importantly get a good bend on the ball and hit the top corner; the shot looks wide for awhile, so the keeper will probably get a late jump on the ball, so if you have good power it should go in. Also try to keep it high, this isn't a line drive, but more of a chip. It's not a soft lob, but the ball should be higher than the ball until it gets to the goal.
To do the Stevie Gerrard rocket, if you're running in the direction of the goal anywhere from 35 meters in just hit it as hard as you can with your strongest foot on the instep. It shouldn't curve at all, just a straight bullet. Keep your head down a lot otherwise you'll guaranteed sky it. If you're lucky the ball with have a dip on it (which is really cool), and if you get real good so that the dip is controllable (I never got to that point, I think the dip is random, but the really good ones can probably control it) then obviously try to have it dip right before it hits the bar. The best dip I ever had was on a bullet from about 25 meters out that looked for sure to be going over the bar about 5 meters to the left of the keeper and then it just instantly dipped and hit the bar.
But yeah, the side of the foot should be used almost all the time. As my first competitive soccer coach always said "placement over power"