<< "fsck" Pretty much an automatic R rating I think. >>
Depends upon the use. If shouted just for an obscenity, such as if somebody gets hurt, scared, mad, ect then you can get away with one or two utterings of it and still get a PG-13 rating. When you start actually using it as a verb, or a noun, then you get into trouble. Example - Telling someone to "Go 'eff themselves" = R rating. A man telling a woman that he wants to funk her = R rating.
It's all in context.
That and the MPAA is the most retarded group of jackasses second only to the RIAA.