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 $58.69   11.411%   $65.3871   $65.39   $65.40   $65.40  
  Purchase #2 $249.92   4.263%   $260.5741   $260.57   $260.55   $260.60  
  Purchase #3 $190.63   5.208%   $200.558   $200.56   $200.55   $200.60  
  Purchase #4 $91.49   11.852%   $102.3334   $102.33   $102.35   $102.30  
  Purchase #5 $29.87   3.531%   $30.9247   $30.92   $30.90   $30.90  
  Purchase #6 $16.36   11.661%   $18.2677   $18.27   $18.25   $18.30  
  Purchase #7 $122.93   13.592%   $139.6386   $139.64   $139.65   $139.60  
  7 purchase totals = $759.89       $817.6836   $817.68   $817.65   $817.70