being a left arm orthodox bowler, i am always told to do one or the other. the simple fact is that first of all, it is always tough for a finger spinner to be an 'attacking bowler' as such, as they are not able to gain as much purcahse on the ball as a leggie. second of all, there are legitimate reasons for both going over and around the wicket to right handers.
going over the wicket is an effective tactic if there is little turn in the pitch, as it brings the slip into play moreso than if bowling over the wicket. if the pitch has a lot of bounce in it, then bowling over the wicket can enhance the chance of a catch going to forward short leg. in one day matches, it works as a defensive tactic, while in the longer version of the game, it can be used to reduce width afforded to a batsman who likes to free their arms, or who cannot play the sweep proficiently and attack this weakness.
bowling around the wicket always increases the chances of an lbw or a bowled, and if the ball is turning, is the most effective form of bowling in an attacking manner. the arm ball is usually more dangerous from around the wicket. it also allows movement around the crease to be more effective.
left arm spinners who were/are strike bowlers include bishen bedi, derek underwood (quick, but did take wickets), vettori and giles. remember that just as most people are right hand batsman, even a greater proportion are right arm bowlers and this means that because the pool is smaller to choose from, the number of quality left arm spinners will be less
