It doesn't matter how you format your css. The only two factors (so far as I know) are:
* Readability. Some people like css written horizontally (as you do), while others prefer a top-down standard. I've talked to some that organize their css alphabetically, while others arrange it according to the flow of their html.
* File Size. If you are working with a lot of css, the file size may be an issue for you. If you arrange your code horizontally, the file will generally be smaller in size than if you did it in a top-down style.
At least that's what I do with an external style sheet. If I have a few styles in the <head> of a page I'm more likely to consolidate things onto one line. In either case, readability and logic is key.