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 $17.25   4.344%   $17.9993   $18.00   $18.00   $18.00  
  Purchase #2 $226.81   7.683%   $244.2358   $244.24   $244.25   $244.20  
  Purchase #3 $213.80   3.753%   $221.8239   $221.82   $221.80   $221.80  
  Purchase #4 $130.71   12.364%   $146.871   $146.87   $146.85   $146.90  
  Purchase #5 $28.39   11.308%   $31.6003   $31.60   $31.60   $31.60  
  Purchase #6 $51.46   9.331%   $56.2617   $56.26   $56.25   $56.30  
  Purchase #7 $193.54   5.243%   $203.6873   $203.69   $203.70   $203.70  
  7 purchase totals = $861.96       $922.4793   $922.48   $922.45   $922.50