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 $58.82   12.617%   $66.2413   $66.24   $66.25   $66.20  
  Purchase #2 $27.90   10.210%   $30.7486   $30.75   $30.75   $30.70  
  Purchase #3 $163.88   3.659%   $169.8764   $169.88   $169.90   $169.90  
  Purchase #4 $20.85   10.274%   $22.9921   $22.99   $23.00   $23.00  
  Purchase #5 $227.34   10.480%   $251.1652   $251.17   $251.15   $251.20  
  Purchase #6 $85.37   10.550%   $94.3765   $94.38   $94.40   $94.40  
  Purchase #7 $22.81   8.795%   $24.8161   $24.82   $24.80   $24.80  
  7 purchase totals = $606.97       $660.2162   $660.23   $660.25   $660.20