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 $59.35   12.385%   $66.7005   $66.70   $66.70   $66.70  
  Purchase #2 $203.51   10.533%   $224.9457   $224.95   $224.95   $224.90  
  Purchase #3 $6.66   9.509%   $7.2933   $7.29   $7.30   $7.30  
  Purchase #4 $242.42   8.828%   $263.8208   $263.82   $263.80   $263.80  
  Purchase #5 $188.65   3.733%   $195.6923   $195.69   $195.70   $195.70  
  Purchase #6 $130.90   4.171%   $136.3598   $136.36   $136.35   $136.40  
  Purchase #7 $125.25   3.303%   $129.387   $129.39   $129.40   $129.40  
  7 purchase totals = $956.74       $1,024.1994   $1,024.20   $1,024.20   $1,024.20