For poetry, while "centering the block" would be the best way, I set it with a larger left margin (2em, when the normal indent is 1em).
If at some point you get a reader that supports "auto", you can just change the margins to "auto". Or, you can set "auto" margins right now, and 2em padding. A reader that does support "auto" shows it properly centered, while a reader that doesn't will show the "second best" option
