Hello Kitty:
It's always better to prevent mistakes than to fix them later. But if that's not easily done as Brian and Steve point out, an alternative is to police the accuracy of the data. A GridView report could be run at the end of the day to look for specific problems (i.e. blank salesperson codes). Of course, this approach only works in certain environments. If you enter Orders and invoice them the same day, it's not going to help because it will be too late to correct mistakes.