When is the right time to file charges on a suspect?

I know the generic answers,"When there is enough evidence, or when the DA decides" I am looking for a more thorough response. For instance, charges can be filed with just PC, probable cause, but in many cases that is no where near enough evidence to support a conviction. In the end what make the decision final? Do DA's only tend to take "winners" to court, or do they act with ethics? For instance, if you have a case and you speak to a suspect. With the evidence you have probable cause, but after speaking to the suspect he has a legit and plausible explanation to explain the evidence that points towards him. Now you do have PC, so that is enough to file charges and let the courts figure it out. But still you have this explanation that is legit, plausible and innocent. Don't investigators and DAs have a moral and ethical responsibility to file on a person that they have no or few doubts did the crime. The person could be telling the God's honest truth, so would it be fair to force this person to prove his innocence? We are supposed to be innocent until proven guilty. Right? When legit alibis and explanations come into play, do DAs and investigators have an obligation to prove or disprove these statements before filing charges? What if they can do neither? Should they play it safe and not file charges? Thanks all in advanced for you answers.