Hi Lauren,
Good question. I generally try to stick with the "in-line" type modification when possible. However, I find the custom code can sometimes be necessary or easier in some case - though there isn't a hard and fast rule that I'm aware of. Here are some example use cases I use custom code for:
- If I want to make a custom CSS change to an element (beyond the typical modification) like the background image reference.
- When I want to make changes to an element that exists many times on the page or even a dynamic number of times. Doing the change one time with custom code can be much easier then visual modification repeated over an over for the same element.
- Changing the functionality of an element on the page.