Yros26 I haven't read anything about this, but I have graded before. The reason you wrote out your formulas is to help the grader understand your thought process and knowledge if you make a calculation error.
What if you type in the formula incorrectly in the Excel calculation? They won't know what you meant to do.
So, I don't know what CAS or SOA say about writing out your formulas, but I think it is beneficial to help the grader understand your knowledge, though practically less necessary than it used to be. If it were me, I would write them out when I had time, and skip it when I don't. But I'd also look to see what CAS and SOA have said about it.