Some commonly accepted style rules are simply wrong, such as the high-school-English rules against initial conjunctions and terminal prepositions. The plain-language school denies it, yet the foregoing rules aren’t entirely bad: their use must be limited to creating rebuttable usage presumptions. As absolute rules, they have been discredited in popular culture, discredited excessively, even the presumption in the rules’ penumbra silenced.
New misguided rules waited in the wings. Today, authorities advance a rule as unfounded as the above positional prerogatives. The vogue rule assigns unalterable sentential roles to “that” and “which.” Recognized writers don’t follow this rule, and, even as policy recommendation, it offends Clarity. The putative rule assigns “that” to restrictive clauses and “which” to nonrestrictive, redundant to comma usage. Redundancy should be avoided: A single distinguishing cue for each distinct response is best for overlearned performances, such as comma recognition.
Don't conclude seemingly arbitrary, picayune rules—even those lacking expert endorsement, like the “and, but” rule and the “to, for” rules, above—are always invalid. At least one rule deserving wide recognition gets scant respect in obscure but esteemed sources listing too many comma uses to learn. According to this obscure rule, a causal adverbial clause beginning with “because” is always restrictive and, “since,” nonrestrictive. In its temporal sense, however, “since” always introduces a nonrestrictive clause, the rule's main payoff. These examples of restrictive and nonrestrictive causal adverbial clauses come from another of my blogs:
Canatella 1 found that the Younger abstention doctrine, which protects ongoing state proceedings from federal court interference, did not compel case dismissal, since Canatella filed before the State Bar served him a case-initiating Notice of Disciplinary Charges.
Gentile's practical import is that in 1991 it had already foreclosed opposing disbarment for frivolous filings because they are believed expressive.
Evaluating ghostwriters, you shouldn’t assume breaking a rule resting only on traditional acknowledgment is error. Clarity trumps pedantry.