This is a good question – to which I can’t offer you a good answer! You might want to submit your question to the comet discussion list and Nick James or another will likely pick it up quicker.
I would say that if your plate scale is around 1 arcsec / pixel or below then you might as well 2×2 bin and use the mono pics for astrometry. I’m not sure what you should do if you are correctly sampling or are undersampling.
I look forward to seeing your colour pics of Wirtanen! I’ve just bought a Hyperstar (Christmas brought forward) to give me a wide field of view for the close approach.
Good to hear you are imaging 64P – it’s nice isn’t it? And 38P is moving to a better position.
David