Yes, it is mandatory. GAFX offers three options to the Producers while posting a Project. If Budget is decided then type can be chosen as 'Fixed'. if not exactly known then type can be chosen as 'Range'. if completely not known then it can be chosen as 'Flexible' and this allows Freelancers to come up with their own proposals.
In all of the above cases irrespective of what has been mentioned by Producer, GAFX provides freedom to Artist to specify any Bid amount that he/she feels appropriate. The same can be explained while doing bidding or during the discussions with Producer later.