As far as I remember, when I was reading a congressional briefing paper (or something like that) from GAO, I seem to remember reading that when someone requests a FOIPA request on A-files, the files are moved to the place servicing the request (broken files are aggregated before the request is serviced). Once the service is completed, the files are supposed to go back to the file repository that USCIS maintains, but that breaks down sometimes.
What I am saying is, if your case is OK and there are no complications, I would wait until I get my N400 done before I would request my file. Remember, the files are PHYSICALLY moved during the FOIPA request/DO interviews etc. I would not want my file displaced before (or while) my N400 (filed or pending). If they misplace your file, you probably would have wait a while until they get hold of it. A more viable scenario is, while your FOIPA request is pending, your file might be requested by the office servicing your request, and lets say you file N400 after that, the FOIPA office will probably not release your file until they are done or some variation of that.
If the only reason you are requesting your file is curiosity, the wise move would be to hold on until you are done with N400. Why take a chance? In this case, "fortune favors the brave" does not hold true, "fortune favors the patient" is more apt.