Currently the category with the lowest ID gets used in the URL when assigning more than two categories to a post.
Would be nice to be able to choose which category gets used in URL.
I assign 3 categories to a post
1. Bars (ID is 3)
2. Restaurants (ID is 16)
3. Hotels (ID is 7)
With default behaviour of WP, the URL will look like this as "bars" has the lowest ID so gets used in the URL: