In Fallout 1, I wouldn't go higher than Charisma = 2 (which would be if you chose Gifted as one of your traits; otherwise go with 1).
In Fallout 2 however it depends. If you want to go through the game without followers (which makes the game much harder, IMO) than Charisma is basically a dump stat yet again. If you do want companions however, I'd go with Charisma 4, then get magnetic personality. This would entirely benefit a standard shooter build, since in Fallout 2, using guns never requires you to max out a single special stat to be good (except Agility, which is the most important stat in the game)