I went with 5 on my current character, with the Bobblehead I could get Local Leader and with a suit, black rim glasses and newsboy cap get up to 10, so that I can pass all persuasion checks. The only reason I'd see to go any higher than 5 is if you want the perks, since with the above items you can get 10 anyway for price reduction/persuasion checks. Grape Mentats also help for price reduction (+5 Charisma and 10% better prices), so I suppose you could substitute those in.