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 $128.62   3.492%   $133.1114   $133.11   $133.10   $133.10  
  Purchase #2 $129.53   6.748%   $138.2707   $138.27   $138.25   $138.30  
  Purchase #3 $39.34   3.796%   $40.8333   $40.83   $40.85   $40.80  
  Purchase #4 $239.25   7.468%   $257.1172   $257.12   $257.10   $257.10  
  Purchase #5 $97.85   13.236%   $110.8014   $110.80   $110.80   $110.80  
  Purchase #6 $178.93   11.287%   $199.1258   $199.13   $199.15   $199.10  
  Purchase #7 $19.60   10.359%   $21.6304   $21.63   $21.65   $21.60  
  7 purchase totals = $833.12       $900.8902   $900.89   $900.90   $900.80