For me it depends on on what I am doing with the notebook, and how much versatility I want the functions defined in it to retain as opposed to how much clutter I'm willing to put up with.

If I am writing a notebook where the computations always involve a few variables with a fixed definition throughout, I set those variables in an obvious section at the top of the notebook, where I can easily change them an do "Evaluate Notebook."

If it's the case that my interest is purely numerical, and always will be, I use Set (=) to define them. If I have an interest in the symbolic solutions, then I often define them as something like values = { a list of rules for the values } and then use Replace when I want to see numbers.

If I expect that I will at times want to use the functions I'm writing with different values for the parameter, then I will pass it specifically as an argument.

It's all a bit of a tradeoff: To many variables in global scope make a notebook and the functions in it less versatile, and as usual global scope can lead to surprises. To many variables passed as arguments can lead to a very cluttered notebook. I find I often choose to work in symbols, with the solution /. values at the end to get numbers.