This is probably a stupid question but honestly I have no idea. I plan on making a 2D tile-based game after I know a bit more C#/XNA. Should I take the time to also learn HLSL for lighting processing etc? Let your reply be defined with the following points:
1. I will be only applying shadows to each tile or maybe even every quarter of a tile (not, for example, every pixel).
2. I'm fairly sure games such as Terraria does not even touch the GPU except for drawing the frames.
I feel confident these days processors can easily handle what I have described (each tile shadows only).
Your question makes no sense, but it doesn’t really matter.
In any case, working with shaders is the correct answer. You are guaranteed to be able to do whatever strange thing you are planning to do right now, and even fixed-functionality tasks are easier to do with shaders.
Shaders are also higher in performance than the fixed-function pipeline, so if whatever this mysterious thing you are planning is advanced in any way it will benefit from the performance of shaders.
Not only that, but knowing HLSL is simply a benefit to your future. It is expected common knowledge these days and having experience with only the fixed-function pipeline does not impress potential employers.