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 $124.40   3.777%   $129.0986   $129.10   $129.10   $129.10  
  Purchase #2 $94.56   4.862%   $99.1575   $99.16   $99.15   $99.20  
  Purchase #3 $78.68   7.419%   $84.5173   $84.52   $84.50   $84.50  
  Purchase #4 $111.56   4.326%   $116.3861   $116.39   $116.40   $116.40  
  Purchase #5 $128.26   10.844%   $142.1685   $142.17   $142.15   $142.20  
  Purchase #6 $43.56   8.627%   $47.3179   $47.32   $47.30   $47.30  
  Purchase #7 $206.78   13.706%   $235.1213   $235.12   $235.10   $235.10  
  7 purchase totals = $787.80       $853.7672   $853.78   $853.70   $853.80