We recommend you only use one category per post. If you have more than one it may confuse users.
Take note, categories are only used for posts, not for pages!
Search Engines may think you’re trying to cheat them by giving posts multiple URLs.
If you want to use more keywords, use tags.
Also, never use the ‘uncategorized’ category.
If you have blog editors who forget to set a category (and hence leave the default ‘uncategorized’ category set), rename this from ‘uncategorized’ to ‘general’ or something similar.