Checking the 1099 boxes on all Vouchers that apply to 1099 reporting is the recommendation for tracking this information in order to print 1099's.
However, it is not required if you are going to use only one box for your forms.
When the 1099 Vendor Report and Forms is run with the Include 1099 Adjustments Amounts box checked, it looks for all the Vouchers with the 1099 boxes checked and adds the 1099 Adjustment amount from the Vendor record into the calculation as well. Thus, it is possible to have no Vouchers with 1099 boxes checked and an amount equal to the amount you want on the 1099 entered in the 1099 Adjustment box on the Vendor record. The system will add the $0 from the vouchers to the $ amount from the 1099 Adjustment box and if the amount is greater than the threshold, will include it in the report and allow you to print the 1099's.