All of us remember the great shot we hit that landed right on the line. The pros seem to be able to do this more often than we can. However, I have always believed that many, if not all, of the shots the pros hit that land on the line, are actually by mistake.
The exception to this might be on the serve since the player who is serving controls all aspects of the ball being hit (using the toss). Most pros can serve accurately enough to hit the lines regularly. The game has gotten so fast that even the pros give themselves plenty of margin on their groundstrokes, volleys, overheads and return of serve. Often, a ball landing on the line was actually targeted for an area a foot or two from where it ended up.
I would recommend picking a target about three to five feet from a line so that if you miss slightly, the ball will still land in and help get your opponent in trouble. As I mentioned before, if you get confident that your toss will be where you want it, you can be a little more aggressive with the target on your serve.
Don't forget that a good serve right in the middle of the box will likely jam your opponent and be very effective. This type of target will accomplish the high margin idea and can still be an offensive tactic.