I have become really good at modifying Oslo or Seattle CSS and clients love the final product but now that I'm learning more about the Rest API and SharePoint Add-ins I wonder if it is a good approach... What I do is I choose the layout that fits my needs, then I create parts in HTML with JS and CSS then upload it to a document library and then I use the Content Editor Web Part to add it to the site, from there I start modifying the CSS and use the Rest API if needed.
I never touch any master page's file such as the CSS nor .master, my issue is, if this is ok to do it?