valid ? not valid?
I saw a message in this board, from a guy in your situation, and what I remember is that since you has been already approved..... you are OK.... Nobody is going to complain about that... nobody will have interest on that (and I dont think your employer will).
It is a different situation if you break your relationship with your employer after your approval and before some prudential time (the law doesnt specify anything, but it is assumed to be between 6mo to 1year) your employer can notify this situation to the INS.... (some times they dont...)