Is this a correct use case?
Display: flex on a <ul>, and then every <li> has either a pixel or percentage based width.
The main reason I want to use it is so that I can utilise align-items: center (vertical alignment).
It seems to work well, and I can't get it working this nicely with anything else, but it feels wrong because I'm not actually using any flex values on the list items.
Yes, that use is perfectly valid. It just means all the children get their default behaviour, which is flex: 0 1 auto (syntax is flex: <flex-grow> <flex-shrink> <flex-basis>). And that happens to do what you want, which is simply align all the items appropriately. You don't want any items to grow (flex-grow: 0), they should all shrink to equal sizes (flex-shrink: 1) if necessary to fit and you want them laid out according to the content (flex-basis: auto).
I recommend going through the following page, because it explains it quite clearly: https://css-tricks.com/snippets/css/a-guide-to-flexbox/
It explains all the attributes, has examples and links to the most crucial W3 resources when necessary, such as http://www.w3.org/TR/css3-flexbox/images/rel-vs-abs-flex.svg for flex-basis.
