I guess it's a matter of balance? I think about it like, the main point of the free posts [in terms of how the business side of the newsletter works] is to travel widely and find new free subscribers, while the main point of the paywalled posts is to turn those free subscribers into paid subscribers. More of the former can't hurt, so long as I'm doing a paywalled post or two a week to keep up conversions. More paywalled posts probably would alienate free subscribers to the point of quitting, though. (Maybe the other key thing is that every single email, free and paid, has a "subscribe" or "upgrade subscription" button near the very top.)

But it probably also depends on your pitch to paid subscribers -- if you make everything free and ask for subscriptions as a gesture of support, like public radio or something, the balance is going to be different.