my theory is its not interesting to see hand seals being half of the show so they choose not to show them most of the time, the reason why they show sakura do hand seals on a basic jutsu is because a) its the frist time we see her use it. b) it gives the fight the feel that she really is trying her best.
the reaosn why they dont show itachi do seals is to give him a superior feel. the hand seal arent just there for doing the jutsu they also give the show a certain feel when they are used or not used.