It is important to remember that while preparing the budget, you often will be required to report back to the funder on a line item basis. In other words, at the completion of the project, you may need to include a comparison between the budget you submitted with the proposal, and the amount of money you actually spent. Some funders will require that you explain variations of a significant greater or lesser amount.
Above all, the project budget is a document that will live with you for the entire duration of the project. Preparing it accurately, and presenting it as a well-organized component of the proposal can be essential in the success of your fundraising campaign.