Opera and IE (and W3C's CSS2.1 "default" style sheet) do have a default margin-left on <ul>,
FF has a default padding-left on <ul>

so I am used to set ul {margin:0; padding:0;}

but Prince seems to set a margin-left: 40pt on <li> in /engine/style/xhtml.css

so I have to set ul, li {margin:0; padding:0;} to undo the default?

Yes, that should work fine.

(Of course, then the list item marker will extend outwards into the left page margin, unless you add any additional margin or padding to the parent).