I know very little abour Boutique's operation. I assume all of their aircraft are set up with 9 or less PAX seats?

As far as logging SIC time, for Passenger carrying part 135 ops an SIC is always required. (C172 or otherwise) 135.101
You have to apply for and receive the A015 OPSPEC which is the exception to the rule. So yes you can log SIC in a single-pilot airplane. Now as far as where they already have A015, I don't know if they can pick and choose when they want to use it because I haven't read the verbiage on A015 in quite a while.

As far as general logging of flight time, the only times anyone cares what you log are when you apply for a new certificate or type, when you apply for a job, or what you log for recently of experience (currency).

I can tell you that I log my time based on who the company assigns as PIC/SIC, because I don't want to deal with sorting out the ACTING PIC from the sole manipulator. Not that I'll have to ever do that again.