When is the best time to promote players from one minor league level to another? Should I do this at the end of the season, so I can release unneeded players before they count against next year's player payroll? Or should I wait until after Spring Training in the new season?

I promote at end of season. That way their new salary is factored in.

Not sure I'd release a bunch of guys. Some will be FA and leave on their own. Others will retire. You always need minor league fodder.

FWIW I promote almost all players to the next level at the end of the Minor League season. Exceptions are BL prospects who need more work at their current level. When I get to AAA, if the player is a BL prospect, I keep him there. The rest of the AAA guys go down to Rookie. I usually need to do a little juggling but it works. AAA guys who don't become free agents or retire the next season are deactivated and used to fill out any rosters as inactive guys the following season. There's usually not a lot of them. I almost never have to fill out minor league rosters this way and it doesn't take too much work.

This is very helpful guys. Thank you.