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 $125.29   8.702%   $136.1927   $136.19   $136.20   $136.20  
  Purchase #2 $152.61   13.650%   $173.4413   $173.44   $173.45   $173.40  
  Purchase #3 $185.76   7.193%   $199.1217   $199.12   $199.10   $199.10  
  Purchase #4 $70.35   11.814%   $78.6611   $78.66   $78.65   $78.70  
  Purchase #5 $47.74   5.651%   $50.4378   $50.44   $50.45   $50.40  
  Purchase #6 $225.37   5.162%   $237.0036   $237.00   $237.00   $237.00  
  Purchase #7 $107.30   8.276%   $116.1801   $116.18   $116.20   $116.20  
  7 purchase totals = $914.42       $991.0383   $991.03   $991.05   $991.00