Value Based Events for Billing

We have Contracts (without Grants) and Project Costing 9.1.  We have set up value based events for our non-rate based billing items.  This was done since there can be a project and activity tied to this event, whereas with true amount only billing events, there is no tie whatsoever to the project, since it originates in Contracts, and is independent of actuals or overhead accrued.  We have had consultants' comments that the approach is fine, and we are also having alarms sounded that revenue cannot be properly accounted for and that they should never be used.  We have 'separate billing