Let's do some math ..  Rounding examples of 7 random hypothetical purchases:
  Case Tag  
Price  
Sales  
Tax %  
Raw Total  
Incl. Sales Tax  
Rounded  
to Penny  
Rounded  
to Nickel  
Rounded  
to Dime  
  Purchase #1 $240.95   7.733%   $259.5827   $259.58   $259.60   $259.60  
  Purchase #2 $231.26   12.255%   $259.6009   $259.60   $259.60   $259.60  
  Purchase #3 $83.86   13.639%   $95.2977   $95.30   $95.30   $95.30  
  Purchase #4 $219.32   11.568%   $244.6909   $244.69   $244.70   $244.70  
  Purchase #5 $10.95   8.245%   $11.8528   $11.85   $11.85   $11.90  
  Purchase #6 $13.92   6.769%   $14.8622   $14.86   $14.85   $14.90  
  Purchase #7 $116.57   3.235%   $120.341   $120.34   $120.35   $120.30  
  7 purchase totals = $916.83       $1,006.2282   $1,006.22   $1,006.25   $1,006.30