When there is a great amount of swing and bounce playing with a straight or vertical bat position is much safer option than compared to cross-bat and horizontal bat positions.
When a batsman plays straight or plays with a straight bat it is the vertical position of the bat which allows the batsman to control over the swing and bounce of the ball.
Playing with a horizontal bat position is also a safer option when there is no bounce, even if there is great amount of swing it is the position of horizontal direction of the bat which makes the batsman confident in dominating the swing of the ball. And we know that the horizontal direction of the bat at the chest or shoulder level position will not be able to contact the ball properly when the ball bounces enormously to shoulder or above shoulder level position. Bouncing deliveries or bouncy tracks are always a threat to horizontal bat shots.
Finally playing with a cross-bat or playing across the line is really not much safer option when there is a great amount of swing and bounce, it would be ideal to play with a cross-bat position when there is no swing and bounce. It is the position of cross-direction of the bat during the impact of the ball which makes the batsman difficult to control over or dominate the swing and bounce of the ball.
In a Test cricket or some times in limited over matches most of the batsmen chose to play straight or play with a straight bat during initial overs of the match when the ball is new since we know when the ball is new and hard there is a greater tendency of swing and bounce from any kind of cricket pitch in this part of the world. Most of the batsmen will start playing with a horizontal bat or cross bat positions after completion of initial overs (like 10-15 overs) when the ball becomes old especially in Test matches since we know when the ball becomes old and softer it will have lesser tendency to swing and bounce compared to initial overs of the match from any kind of cricket pitches.
Specially in Test cricket if there is a great amount of swing and bounce playing straight would be much safer option than playing with a horizontal or cross bat, if there is only a swing playing with a straight bat or horizontal bat would be the safer options than with a cross bat, and finally if there is no swing and bounce playing all three kinds of shots would be the options to chose.
At the end of the day these are the basic rules to be followed by any cricketer but rules can also be broken if a batsman is really capable of picking the swing or bounce or any kind of delivery, then he could play any kind of cricketing shot in any time and in any form of cricket.
Now a day’s most of the modern day batsmen are trying to do this and have succeeded in playing all kinds of cricketing shots in all forms of cricket during any period of the game like even it may be initial, middle or final overs.