Hi, I'm new. =p

Maybe the reason Sakura was doing the handseals in the battle was because she wanted to psyche out the Sound Nin. Remember how she did the hand seal and didn't actually do the replacement technique at the end? Her plan all along may have been to show the hand seals on purpose so that she could get close to the Sound Nin, and...er...bite him.

And about them not showing the handseals for some moves, yes, it gives away the move, yes, the animators are lazy, and maybe hand seals are only performed to focus the chakra so that doing the jutsu is easier. That way if you're good enough, you can focus enough chakra to do the move without performing the hand seals.

That's just whay I think. =p