You guys are right when you say that a DM can make adjustments to account for the way a player roleplays the encounter, but that's a very 1st Edition mentality (and I like it!) 3.x isn't MEANT to do that however. Using the example of talking to a person whose dog I killed... if the DC to persuade the person to like me is 30, but there's still a chance they'll hate me even if I make that roll, then the DC is set too low in the first place. The whole point of the DC system is that the target number is set based on ALL in game factors, such that the player need to hit that number to accomplish their task. On the other hand if you're saying the NPC might hate me regardless of what I roll ('cause I killed his dog, after all) then you're going against the 3.x system altogether.
Cass: Why don't I get a save?
Lodge: It's a Death Demon. It's fear aura is too strong. You can't roll a successful save.
Cass: Incorrect.
Lodge: What's your Will save modifier?
Cass: +9.
Lodge: The DC to beat this demon's fear aura is a 32, you would have to roll a 23 on a 20 sided die in order to succeed. Now I don't have a math degree, but that's impossible, isn't it?
Cass: A natural 20 is an automatic success no matter the circumstances.
Lodge: But you only have a 5% chance of rolling that 20.
Cass: Do I at least get that roll? I mean, it's in the rules. Is it okay if we play by the rules?
Lodge: (waves for him to proceed)
Cass: Rolls a natural 1
Lodge: OOOooohhhhh!!! A fumble! That means you're completely compelled, and I get to control your character until you snap out of it! Since we're playing by the rules, I know you don't mind.
Cass: Oh shut up, Kevin.