As you say, there is a lot of strategy in the use of your rotation. In a tight race, I've gone from a 5 man to a 4 man in mid- September because I thought I needed to squeeze out an extra win or so and having my number 1 and 2 get a few extra starts gives me a better chance. I've gone to a 6 man when I'm 10 games up or back in September just to get a look at what some other pitchers can do as well as spot starting call-ups. For the long haul most stick with the 5 man rotation because fatigue could lead to injury and this is something that we all want to avoid.
As far as the game use of rotation size, you may opt to play in an era where it was the norm to use a 4 man rotation and pitchers regularly went deeper into games. All up to you.
