If you're writing an entire application in assembly for fun, then, sure, have at it. But if you're writing an entire application in assembly because you want better performance, then I think you're misguided. Write it in C, turn all optimizations on, run it with a bunch of benchmarks, profile it. Look to see where the bottlenecks are. Optimize those, maybe in assembly, or maybe by just being smarter with your C code.
If you're writing an entire application in assembly for fun, then, sure, have at it. But if you're writing an entire application in assembly because you want better performance, then I think you're misguided. Write it in C, turn all optimizations on, run it with a bunch of benchmarks, profile it. Look to see where the bottlenecks are. Optimize those, maybe in assembly, or maybe by just being smarter with your C code.