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 $82.37   12.166%   $92.3911   $92.39   $92.40   $92.40  
  Purchase #2 $223.99   10.554%   $247.6299   $247.63   $247.65   $247.60  
  Purchase #3 $128.90   6.802%   $137.6678   $137.67   $137.65   $137.70  
  Purchase #4 $231.57   6.459%   $246.5271   $246.53   $246.55   $246.50  
  Purchase #5 $63.74   12.709%   $71.8407   $71.84   $71.85   $71.80  
  Purchase #6 $122.83   5.444%   $129.5169   $129.52   $129.50   $129.50  
  Purchase #7 $31.83   6.626%   $33.9391   $33.94   $33.95   $33.90  
  7 purchase totals = $885.23       $959.5126   $959.52   $959.55   $959.40