I think the OP's point was that the usage of slang should be appropriate to the character. I've always been tickled pink by character who use big word incorrectly in an effort to effect intelligence.
I like making up cuss words, and I think that as long as they really sound like angry cuss words, they work.
A sample from Canadian Knights - "Why, you infinsolant bock!"
Seriously, just say Infinsolant Bock in an angry, accusitory tone. You'll feel dirty.
In the one case where we use real profanity, or at least imply its use, we do so by grossly mispelling the word in the dialogue box and then "Bleeping" the word out in the voice-over. An example:
Bildo
"Holy schitt!"
Indy
"Bildo, how did you make that sound with your mouth?"
Yeah, that's right - Indy heard the beep too!
Again on slang, I think it can be beneficial to kind of invent your own slang and expressions in addition to some nice cursing, to help establish notions of culture and history.
"This will put chest hair on your chest hair."
Answering the phone with, "Hello, Ma?"
"Oh sweet merciful freakin' A"
"This does not bode well for the bood."
"Go drown in the lake, you fat baby!"
It's nice to see established expressions, especially if you learn to happily predict a character's response. You get into the setting, and have a better time.
Oh, and for the record - we use "Bitch" all the frikkin time, but always in comedic context.