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 $19.40   8.136%   $20.9784   $20.98   $21.00   $21.00  
  Purchase #2 $156.69   12.432%   $176.1697   $176.17   $176.15   $176.20  
  Purchase #3 $42.56   4.475%   $44.4646   $44.46   $44.45   $44.50  
  Purchase #4 $215.47   11.112%   $239.413   $239.41   $239.40   $239.40  
  Purchase #5 $203.56   6.549%   $216.8911   $216.89   $216.90   $216.90  
  Purchase #6 $156.25   12.161%   $175.2516   $175.25   $175.25   $175.30  
  Purchase #7 $115.72   4.376%   $120.7839   $120.78   $120.80   $120.80  
  7 purchase totals = $909.65       $993.9523   $993.94   $993.95   $994.10