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 $121.50   12.536%   $136.7312   $136.73   $136.75   $136.70  
  Purchase #2 $134.54   11.349%   $149.8089   $149.81   $149.80   $149.80  
  Purchase #3 $111.51   4.638%   $116.6818   $116.68   $116.70   $116.70  
  Purchase #4 $232.63   5.142%   $244.5918   $244.59   $244.60   $244.60  
  Purchase #5 $134.92   8.674%   $146.623   $146.62   $146.60   $146.60  
  Purchase #6 $19.80   3.628%   $20.5183   $20.52   $20.50   $20.50  
  Purchase #7 $181.34   8.449%   $196.6614   $196.66   $196.65   $196.70  
  7 purchase totals = $936.24       $1,011.6164   $1,011.61   $1,011.60   $1,011.60