I'd like to know if that's optional take or not this sign, because i'm known that a few idioms are used to have this sign, thus i want to know why do they use this and how does it affect the sentence ?
一丝不苟 basically means "[even] one thread not careless", referring to meticulousness.
So ordinarily dropping the 一 is not reasonable, and certainly so among fixed idioms.
However, in some non-idiom cases we can drop the 一, such as:
In these cases it's different: we have a measure word (个) and it's possible to sometimes drop the 一.