It's a lot more interchangeable than yaoi, but there's still a lot of dom/sub dynamics in yuri depending on the artist and characters. The proper lesbian terms for it seem to be "tachi" and "neko", but artists will still probably use seme/uke just cos it's more well-known terminology..
^Yeah I heard the use of tachi and neko and switch only once in a visual novel, but I assumed that those were the official terms for yuri pairings. I only hear seme/uke stuff from people who like yaoi.