I'm currently working on a revision to a checkout process in which we're debating the pros & cons of pre-populating certain fields.
The application allows users to select an item for "purchase" and pledge an amount related to the value of that item. This means they can in fact pledge more than the stated amount for the item, or the exact amount of the item. They obviously cannot pledge lower.
The debate is such:
Do we pre-populate the form field with the amount of the selected item or leave it blank to encourage some percentage of greater pledge.