I just signed an IFA at the very end of the season. Minor leagues are already starting playoffs. If I wait to assign him, say after the ML regular season ends, will he avoid wasting a year of development?

I think if you wait too long to assign him you lose him.

I have no idea if the player would avoid using up a year of development, but on the other hand, if you wait till next season he's going to start one year older without having played any games. IFAs are already generated further along in their development than HS or College players, just guessing here but maybe for this reason that they may start playing very late in the season. I have always just put them on whatever team has games still left to play.

Also as dschang says, if you acquire any player but leave him unassigned for 30 HBD days (10 real life days) the system auto-releases him. You probably don't want that.

IFAs develop exactly as if they were drafted when they were 18. So 21-year-old IFAs are almost done, but 18 year olds have a lot of development left.

Because of this development pattern, I would assume there is zero value in holding an IFA unassigned.

Agree with the above -- assign him to whichever minor league squad is in the playoffs. Once that team is eliminated from the playoffs or wins the Minor League World Series, promote him shortly after to get some development.