If a journal has optional costs, is it bad form not to pay them?
I'm not sure which discipline you are referring to, but I have published my articles in social and health science journals. Most journals offer options at a cost (optional Open Access, color reprints, etc.). If you do not want or need these options, there is no reason to feel that it is wrong to decline them. I have never paid these extra costs. Journals that are not strictly open access make their money from subscriptions, not from the authors. The additional costs are mainly to cover optional features that would make your article more costly to publish than other articles without these features. So, no worries!