Bill,

You are asking about denormalizing the database and this is a common practice. The rules I follow regading this are;

1. Always make the actual calculation the "valid" value for the data so that whenever the summary field and the calculation disagree the summary will be updated from the calcualtion.

2. Provide a method for the user to "repair" the summary data by recalculating it. i.e. a button beside the field in the data entry form to recalc the summary.

3. On important reports (like financials and such) use the calculation and not the summary field