If you remember your high school geometry, you'll know that the distance from baseline to baseline is considerably longer when measured from corner to diagonal-opposite-corner as opposed to straight down the sideline. Add to this the fact that the net is suppose to be six inches lower in the centre of the court than over the sidelines (granted it is rarely as much as this on recreational courts), and it is quite obvious that hitting cross-court is safer than hitting down-the-line.
Furthermore, if your opponent’s cross-court shots are deep and/or powerful, trying to change the direction of your shots carries the risk of hitting late and spraying the ball wide. And, if you hit down-the-line without having worked your opponent out of position, they can easily have you scrambling from one side of the court to the other with a safe cross-court shot of their own.
So be patient and wait for a low-risk opportunity. Only if you sense your opponent is not recovering adequately, or if they give you a short ball, should you consider taking a chance and hit deep down-the-line. See you on the court!