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 $8.41   10.669%   $9.3073   $9.31   $9.30   $9.30  
  Purchase #2 $14.79   5.206%   $15.56   $15.56   $15.55   $15.60  
  Purchase #3 $105.87   3.347%   $109.4135   $109.41   $109.40   $109.40  
  Purchase #4 $104.94   4.865%   $110.0453   $110.05   $110.05   $110.00  
  Purchase #5 $122.60   10.652%   $135.6594   $135.66   $135.65   $135.70  
  Purchase #6 $133.88   6.186%   $142.1618   $142.16   $142.15   $142.20  
  Purchase #7 $93.78   3.853%   $97.3933   $97.39   $97.40   $97.40  
  7 purchase totals = $584.27       $619.5406   $619.54   $619.50   $619.60