I don't agree that nothing should be outstanding for over thirty days.  For clients that are lowballing you, yes, prompt payment is important.  For long standing clients on large commercial orders, waiting a few months is reasonable if your fees are high enough.  I've waited a year for payment on some large deals, and I really have no problem with it.  These are on deals that I am charging a good premium over my regular search fee.
I don't think you should consider payment schedules in such black and white terms, but keep in mind the time frame of closing on the job you are working on.  This may not be true with the national vendors, but with the local folks I work with, we have a partnership and try to work together in a way that all of us are making money.