I have a coworker who has a habit of CCing our common boss on nearly all email correspondence. The boss is hands-on enough to know what we are talking about but he is also cool and practical enough to not micromanage and I am almost 100% sure that her CCs do not get much of his attention considering his work load, his email traffic, and the relative relevance of her email content. Even when I initiate an email thread and she is the only recipient, when replying she would add him in CC. I tend to be minimalist and considerate of my manager's work load and prefer to not get him involved if things can be resolved between a coworker and me alone. Frankly, I think every smart manager (which I consider mine to be) would prefer this mode of operation.
When replying to her, my obvious choices are to
Reply all, which includes the boss, meaning I am adding to the clutter in his inbox and just following her (in my opinion needless) precedent of CCing him (while I do not believe he wants to be on it), or
Remove him from CC but my concern is then he might think I never responded to her reply.
How should I proceed in this type of situation where undue monitoring is being requested not by higer ups but actually peers and I suspect managers do not want to be involved?