Rat, you DO need to be a pilot to taxi an aeroplane with the intention of flying it. Just log what you're supposed to, block time. Then if your company gets audited and someone has a look through your flight and duty times they're not going to start asking why you're not logging the whole flight.
There used to be a company in NZ that did it all backwards. Timed from off blocks to on blocks and took 0.3 off to arrive at an MR time. Problem was they were doing short sectors and sometimes the resulting "flight time" suggested that they were flying F/A-18s when they weren't.