Is it bad to ask the editor about the status of a submission on seeing that referees' reports were submitted a week ago?
Once reviewers reports have been returned, the editor has to assess the reviews in light of the paper and provide the author with indications on how to proceed. Exactly how this come through will differ between fields and editors. The main point, however, is that a certain amount of work is usually needed to process the reviews before passing the manuscript with comments back to the author. Since editors also handle, sometimes, quite large volumes of papers in parallel, your manuscript will be placed in a queue and the editor will likely take each manuscript in order. In addition, many, if not most, editors do their editing in parallel to ordinary faculty jobs which may at times further limit their available time. It is thus reasonable that this process may take several weeks. In "my" journal we as editors have three weeks for this process before being reminded by the electronic submission system that the manuscript is "due". We often make this deadline, but not always depending n workload.

So, one week is definitely too short a time to expect a response. Three weeks seems to be reasonable in my field but I would maybe allow even additional time to pass. It is probably good to talk to your peers who may have experience with the particular journal and also your field to get feedback on what is considered reasonable.

Added to clarify a comment: it is never "bad" to contact anyone, but it is only fair to allow a a "reasonable" (in the sense of the field and specific journal) amount of time to pass since the early contact will not likely yield anything productive. What, on the other hand, is bad is rather the attitude with which some contacts are made. Insights into appropriate time frames will help to keep communications as fruitful as possible.