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 $9.54   10.301%   $10.5227   $10.52   $10.50   $10.50  
  Purchase #2 $12.27   9.362%   $13.4187   $13.42   $13.40   $13.40  
  Purchase #3 $77.56   12.571%   $87.3101   $87.31   $87.30   $87.30  
  Purchase #4 $137.61   6.597%   $146.6881   $146.69   $146.70   $146.70  
  Purchase #5 $211.78   4.347%   $220.9861   $220.99   $221.00   $221.00  
  Purchase #6 $45.66   10.659%   $50.5269   $50.53   $50.55   $50.50  
  Purchase #7 $116.71   7.426%   $125.3769   $125.38   $125.40   $125.40  
  7 purchase totals = $611.13       $654.8295   $654.84   $654.85   $654.80