Awesome to see all the replies in this thread... My original question was less about the saving throw bonus - Skip's response there is pretty good as a guideline IMO.
After reading these replies and thinking on the matter, I'm curious how Charm Person scenes have played out for folks, more on the RP side.
You are in a dungeon and come across a room of orcs. Before combat begins, you throw up your hand and say "wait a second, I wish to speak with you". You then cast Charm Person. The orcs obviously see you casting a spell, let's say one becomes charmed. He suddenly believes you two are good friends. Do you actually say "Mogrosh, my good friend how have you been these years?" and he will believe you have a long standing friendship? This seems like it would need a Bluff check. Or, does Mogrosh just feel you are a special trustworthy human and try to convince the other orcs to give you a chance to speak, while smiling at you?
You are in a barracks and your commanding officer is a real jerk. He has always had it out for you and gives you some crap job to perform today. You cast Charm Person... perhaps prior to doing this, you Bluff and say that you are going to cast Light or Mage Hand or another random spell. He has no Spellcraft ranks, but is suspicious why you would be casting a spell. You successfully Charm him. Is he more apt to believe that the previous spell was Light when no Light effect came to being, and he is suddenly thinking that you two are best friends, even though in his memories he knows for the past year he has made your life a living hell?
You are bartering with a merchant in a town where spells like Charm are forbidden. You are frustrated at his high prices and tell him you want to cast Detect Magic or Identify (Bluff again) on the item. You instead cast Charm and pretend that you are inspecting the item (Bluff again). You finally look up and say "Hey are you sure I can't get this at cost?". The merchant agrees and you make off with the item. Later the spell expires - does the merchant look back and wonder what the hell he was doing? Is he suspicious enough to wonder if you cast Charm at him? Maybe he gets a Wisdom check to ask around and see what others think about his strange behavior? The way I've played this, is if the player bumps into the merchant later, he just shakes his head and says "lad, you got the best deal I've given this year, I ain't sure how ya did it..."
Any other scenarios would be welcome. Handling the Psionic Charm is much easier since there aren't verbal and somatic components, just the manifesting effect (maybe a chime sound or wind, etc). However, with the spell, assuming you aren't doing Silent/Still, it seems like folks would be more suspicious of their sudden friendship with you, and that using Bluff would be necessary to redirect that suspicion. Obviously charming someone alone is better, since others can't nudge them along by talking about the fact that you DID just cast a spell.
