General rules:
Do you need an answer or do you just want an answer? If there is an actual need (e.g. you need to know for other recruitment processes/commitments), then it is okay to ask. If it is just to give you some reassurance, I generally say avoid asking and trying to sit it out.
If you know many other candidates have heard back and you haven't, it is okay to ask when you may hear back. If it sounds like many candidates are waiting to hear back, then I would just be patient and wait.
If a firm provided a timeline as to when you would hear back and it is more than 1 week beyond that deadline, then you could ask for an update on timelines.