How to Normalize Frequency in MATLAB
The interval resolution should be determined according to data you want to analyze.

It is equivalent to the question, what magnification of your Micro Scope would you use?
Well, it depends on the size of the things I want to look at.

If your data within the sampling frequency you set has different energy in each frequency bin and you'd like to see it, use small intervals.

Though,
The real question, how do you intend to use the Frequency Vector?