When I’m creating WP sites for clients I have a strong urge to break down my CSS files into separate areas. For example, reset.css, typography.css, and so forth. The only thing that stops me is knowing that it negatively affects load speed. In a number of themes I’ve purchased I have found this to be quite common. Is it considered good practise to do this in the themes I sell?
You could just do it this way and then compress all css files into one min.css at the end. Therefore you can work as you are most comfortable and once compressed the page speed issue will not apply.
Using this method, you will need to included an uncompressed example head section in your download package and also a section in your docs explaining how to replace the compressed css with the uncompressed if your buyers wish to make css changes. If not done right though it could lead to lots of support requests.
This is the way we work it in our TF & CC files because we usually have reset, main & skin stylesheets, plus all of our js functionality is developed as plugins for ease of reuse & hence lots of individual import statements. Then we compress both css & js for the live sites and include the above instructions & example for any buyer who wants to get their hands dirty in the css / js code.