There's no hard and fast rule for styling a project.
I always choose which way to style my projects based on what that project is. If its small scale, only me working on it and relatively static then I might in-line CSS. If its a large scale component based web application then I'll use SASS with some packaging & optimising tool. If I can use H2 server push I might keep everything separate and only serve it up as its needed.
I think the method you decide on depends on what that project is, who will be working on it, how its intended to be consumed and what level of performance you deem suitable.
As is becoming more and more obvious styling a project is about more than what tool you use, I think the developer needs to understand the aim of the product and how its intended to be consumed, mobile (poor 3g) / or desktop (high speed fibre), after understanding the target use case and the network restrictions that may apply then you should consider what way to go about styling your project.
Just my 2 cents :D