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 $246.49   4.111%   $256.6232   $256.62   $256.60   $256.60  
  Purchase #2 $100.73   3.663%   $104.4197   $104.42   $104.40   $104.40  
  Purchase #3 $51.43   11.326%   $57.255   $57.26   $57.25   $57.30  
  Purchase #4 $198.82   9.608%   $217.9226   $217.92   $217.90   $217.90  
  Purchase #5 $95.80   6.217%   $101.7559   $101.76   $101.75   $101.80  
  Purchase #6 $130.70   9.291%   $142.8433   $142.84   $142.85   $142.80  
  Purchase #7 $71.35   6.392%   $75.9107   $75.91   $75.90   $75.90  
  7 purchase totals = $895.32       $956.7304   $956.73   $956.65   $956.70