I've stayed away from this discussion in the past and I'm amazed at all the answers that come up. I think a log book should be about showing your experience in an accurate and honest way. IMHO what "can be logged" doesn't always give the accurate picture. For example, if a professional pilot goes and gets a SIC type in a large business jet where he/she is a required crew member and gets a few hundred hours a year performing that duty he logs that SIC time. If I was to log the gear and radio time (yes I do more than that) on the company Cheyenne as SIC that would in essence be devaluing the time logged by example A.
You'll never get a consensus from a forum like this on a technical question open to a lot of interpretation. Having said that, here's what I do...and I'm not going to argue the rights and wrongs of it, (although I'll happily take opinions from those who would like to share them!)

This is not meant to be a legal interpretation by any means, it's just what I do. (and I don't believe it's illegal in any way)
I fly for a part 91 operation, as a contractor, SIC if you will (but I call it co-pilot or gear and switch bit**). The FAA does not require two pilots on any of their aircraft, their insurance company does. (with passengers aboard)
I log my PIC time, where I am on the company paperwork and flight plan as PIC. Happens once in a while but not much.
If I'm co-pilot I will only log flying time (sole manipulator of the controls). I don't log this as PIC or SIC but in a separate column in the electronic log book so that a potential future employer can decide whether or not it's of importance to them in considering me for a job. It's easily distinguishable and I don't include it in my total time. I understanding the whole "flying PIC / acting PIC" point of view but a lot of senior captains at different operations I've hangar flown with don't think this type of PIC logging is a good idea as it can be a little misleading. The reason for this is when the boss is having his lunch in the back on an empty leg and I have the controls, (babysitting the AP) am I making any PIC decisions? I have been told that during an interview if 1/3 of my multi time was found out to be of this type of flying and it was simply included in SIC time and not separated out it could make the rest of my log book a little suspect should the interviewing party not agree with the SIC definition I'm using.
If I'm just working the radios and throwing the gear lever, I don't log at all. I just happily take the few piasters they pay me!
I fully understand that it might be easier for me to take this stance than some people staring out since I have my ATP and 135 mins courtesy of 16 years of recreational flying.
I just feel that it is the most honest way to record this time for the purpose of giving a potential employer a legitimate picture of my experience. So far it's worked out great, flying with some great Capt's and had some really nice trips.
Merry Christmas everyone!