The FARs in this case state 2 things. They state what you MUST log and what you MAY log (for the purposes of 61.51 only). HOWEVER, the "musts" and "mays" are not all inclusive. 61.51 repeatedly uses the word "must" but it is not used in reference to recency, for example. You "must" also have an acceptable amount of recency logged to be a PIC with passengers, but that is not referred to, here because 61.51 is intended to be referred to when judging logbook entries for aeronautical experience in certification. Also, it never states when you "may" log "Total Time". I can't find any reference of "may not" in the FARs for logging total time. I think this is purposeful in the spirit of the FARs because aviation is ever changing. You can certainly log anything you want as you see it as long as you are properly rated. It can be argued that right seat time in a C90 under Part 91 is not good for total time simply because you are not a required crewmember, but if you have a company that values that time, what is stopping you from logging a column with right seat C90 and total? The FAA certainly isn't. They do not care. They only care about what is required to obtain, maintain, and exercise the privledges of the certificate you hold in a safe manner. Some hiring officials do care, though and that is where the stigma behind this comes from. Some people maintain that because you are not "required" that none of it counts. They are certainly entitled to their opinions for the purpose of hiring you. This is why I have a log of B200 right seat time, but it is separate from all of my other time. The FARs don't say I can't keep a record of this. I mean, 1 pound separates the requirement for a right seater and the certification of that airplane. How do you quantify the difference?!?! You don't. You log it but you just use it as what it is. Is is worth anything? Only to those who want it, but there is certainly nothing that says you can't log it. The FARs are specific, but at the same time only inclusive to the FAA's needs.
I'd like to add that if the FARs were regarded as all inclusive, not one Space Shuttle Commander would be able to log any of their time in the Orbiter as it is not specifically regarded as permissible flight time under certification nor recency. For carrying passengers, however, they'd have to have at least 3 "takeoffs" and landings within the previous 90 days...if the Orbiter was even certified by the FAA as an aeronautical aircraft.