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 $36.67   8.573%   $39.8137   $39.81   $39.80   $39.80  
  Purchase #2 $25.95   7.659%   $27.9375   $27.94   $27.95   $27.90  
  Purchase #3 $125.83   11.223%   $139.9519   $139.95   $139.95   $140.00  
  Purchase #4 $102.60   5.355%   $108.0942   $108.09   $108.10   $108.10  
  Purchase #5 $137.92   5.652%   $145.7152   $145.72   $145.70   $145.70  
  Purchase #6 $103.86   4.116%   $108.1349   $108.13   $108.15   $108.10  
  Purchase #7 $18.60   11.360%   $20.713   $20.71   $20.70   $20.70  
  7 purchase totals = $551.43       $590.3604   $590.35   $590.35   $590.30