If it was up to me I wouldn't mind as a rule to write always # when the passage is ascending, and b when the passage is descending. But I have many doubts about it:
1- Why baroque or classical composers respected the rule of lowering the VII grade (and not augmenting the VI grade, for example in Cmajor A-Bb-B instead of A-Ab-B), and the rule of raising the IV grade (and not lowering the V, for example G-F#-F instead of G-Gb-F). Probably they did it because the two nearest alterations in key are F# (for G major) and Bb (for F major), but is it a good justification today?
2-Another rule they respected, for example in A minor they would never do: A-Ab-G-Gb-F, because the VII and VI in A melodic minor is always G# and F#. So they would go: A-G#-G-F#-F.
3- Why composers such Mozart would respect the rule above, but in all the other cases would write both # or b for ascending or descending chromatic passages? There was any rule or was random choice?
4- If today I write something in baroque style do I have to respect the rules above?
I forgot to say that I am talking only about situations where chromatic notes are used in the melodic line just as ornament, embellishment, without changing the function of the chord.
What do you guys think?
thanks

I'm sure someone will be along to talk about temperaments and enharmonic equivalence (or lack of).

There are two methods to write a chromatic scale. Some people refer to the two methods as "harmonic" and "melodic."

Harmonic:
- Write the tonic and dominant notes once.
- Write the other scale-degrees twice.
- Add the necessary accidentals so that the notes proceed upward/downward in semitones.

Melodic:
- Use sharps in ascending and flats in descending.
- Be sure not to use the same letter name three times in a row.

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Indeed, the "harmonic" chromatic scale has much to do with tonality and temperament. For instance, in the key of C major, F-sharp and B-flat are much closer to the tonic note C; they hint at the closely related keys G major and F major. Their enharmonic equivalents, G-flat and A-sharp seem much farther away from the tonic note.