This is the bottom line, if INS finds out that you have EAD (or eligible to get) and either you or your sponsoring employer are not working or does not want to work togather, INS will most likely deny the petition.
Theoritycally, GC job is for future employment. This would mean that it should be OK to join after 485 approval. But, INS wants to make sure that this whole process was not a sham or fraud. Both of you really intended to form a employer-employee relationship. How will you show this ? by joining the employer as soon as you get the authorization to work there (meaning EAD)... remember, bad economy... no project now but will be in future etc. does not help because at the time of filing labor certificate, employer says under outh that the position is vacant and available for immediate filling.
How will INS know ? If they send you RFE (if you are not working for sponsoring company, you may get one) .....