Does a prosecutor ought to interview the victim if physically available and the suspect if physically available to file criminal charges (in the US)?
There is no requirement to interview the victim and/or the suspect prior to filing charges. Often statements are taken from the parties involved/witnesses by police and presented to the District Attorney's office as evidence. However charges can be filed without either party being interviewed, especially by the DA. This can often be the case in things like domestic violence cases, where the victim refuses to cooperate and the perpetrator refuses to talk ("lawyer up" or invoke 5th amendment rights).
Charges can be filed based on circumstantial evidence of the crime (in the example, marks on the fist of the perpetrator and injuries to the victim, along with proximity). The police will try to interview the suspect and/or victim, but usually the prosecutor does not get involved at this point until charges are filed and the defendant has retained a lawyer (or declined one).
If a witness makes a materially important statement to a prosecutor, and the witness later makes a contrary statement, the prosecutor becomes a fact witness to the original material statement. In most situations, a person cannot be both an attorney and a fact witness in the same case. So if the original statement is important to the state's case, a DIFFERENT prosecutor will have to take over the case so the initial prosecutor can be a witness. This problem can be prevented by having another competent adult present during witness interviews so that other person can serve as the fact witness if necessary.
Intent is an element of many criminal offenses. In the example of domestic assault, the prosecutor must prove beyond a reasonable doubt that the defendant acted intentionally, knowingly, or recklessly. A person cannot negligently or accidentally commit assault. So it would be WISE for SOMEBODY to interview the alleged victim, because you are going to waste a lot of time and money prosecuting a DV case if the alleged victim's testimony is that any injuries were the result of consensual mutual combat.
Probable cause to believe an offense was committed can be developed without interviewing any witnesses. For example, if a 13-year-old girl turns up pregnant, any doctors or nurses helping her would have an affirmative legal duty to report child sexual abuse to law enforcement. A child that young cannot legally consent to sexual contact and Sexual Assault Of A Child is a "strict liability" crime (which means there is no intent element of the crime and it doesn't even matter if she lied about her age.) The pregnancy itself is evidence of the crime and a search warrant for the suspected father's DNA may be all that is needed for proof beyond a reasonable doubt.