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 $16.50   11.748%   $18.4384   $18.44   $18.45   $18.40  
  Purchase #2 $92.42   10.443%   $102.0714   $102.07   $102.05   $102.10  
  Purchase #3 $139.60   3.448%   $144.4134   $144.41   $144.40   $144.40  
  Purchase #4 $43.42   4.872%   $45.5354   $45.54   $45.55   $45.50  
  Purchase #5 $197.67   7.117%   $211.7382   $211.74   $211.75   $211.70  
  Purchase #6 $125.58   9.680%   $137.7361   $137.74   $137.75   $137.70  
  Purchase #7 $5.82   5.743%   $6.1542   $6.15   $6.15   $6.20  
  7 purchase totals = $621.01       $666.0871   $666.09   $666.10   $666.00