If you've done anything that was anti-NCR in a big way, all your good deeds don't matter for shit.
To be specific, if you're at like, Dark Hero or Wild Child or something, you're at risk. I wasn't attacked by the roaming ranger/MP in the wastes but I went to the embassy on the Strip and a random MP attacked me.. while I was turning in a positive NCR quest to the Ambassador!
Helios One Quest spoiler:
[spoiler]The fastest way to see how this works is to vaporize everybody at Helios One after getting a good NCR rep. You'll almost instantly become neutral or worse.[/spoiler]
BTW Charisma is practically useless. Speech covers a lot of face checks, but Science, Explosives, Guns, Survival, Barter and more are also factors. Charisma Perks are also weak. In Fallout 3, Animal Friend was available as fucking Yao Gai start spawning in a level-scaling, based on your proximity system. New Vegas has much less interesting and dangerous animal encounters, including most of them being fixed.
Charisma is very useful for the "Let My Followers Do All The Fighting" combat style and that's about it.