It does matter in some cases.
You can look at some of the discussions the semantic-ui creators and contributors had about the word-order issue back when 1.0 was being released: https://github.com/Semantic-Org/Semantic-UI/issues/396
Tip: Be careful to follow the word order especially when you are using grids.
From the documentation:
Grids include many variations for adjusting things like vertical or horizontal alignment, text alignment, or default gutter sizes.
Some multi-word variations, like left floated or right aligned are word-order dependent and require you to use adjacent class names.
A rule of thumb: Never mess up the word order for a multi word class (right floated, left floated).