Nah, that is what the turn is for. The rock will "curl" in the direction of the turn, if you spin the rock counterclockwise as looking from the throwing position the rock will curl to the right, clockwise will make it curl left. The amount the rock will curl depends on the weight you throw the rock, a faster thrown takeout curls less then a draw shot.
The sweeping is done for 2 reasons, it will make the rock run straighter, and it makes the rock move farther along the ice. So if you throw a takeout and you are curling too much they will sweep, or if the rock is coming up light on a draw, or any combination of the two.
The amount of spin you want to put on the rock in a shot varies abit depending on the ice conditions but it is usually between 1.5 to 3 complete rotations along the entire length of the ice, if you spin it more then that it actually reduces the amount of curl and less then that and the curl can be too extreme or it can actually stop spinning all together and that screws up the shot royally.
This is why you watch the game, much like pool there are rare moments of insane shots.
http://www.youtube.com/watch?v=qa-1_bNN0bY
http://www.youtube.com/watch?v=1kE5QzOprns (this shot in particular is nuts)
http://www.youtube.com/watch?v=xfVx1-emCV8 (drag on a rock, very similar to alot of pool shots)
Usually by the time you get to near the final rocks of each end they are always trying to make some pretty tricky shots. Unlike pool the game is never as controlled and you end up playing the equivalent of a 3 rail kick quite often.