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 $30.27   8.202%   $32.7527   $32.75   $32.75   $32.80  
  Purchase #2 $121.37   8.612%   $131.8224   $131.82   $131.80   $131.80  
  Purchase #3 $214.88   8.602%   $233.364   $233.36   $233.35   $233.40  
  Purchase #4 $110.75   8.681%   $120.3642   $120.36   $120.35   $120.40  
  Purchase #5 $229.34   10.859%   $254.244   $254.24   $254.25   $254.20  
  Purchase #6 $111.53   5.653%   $117.8348   $117.83   $117.85   $117.80  
  Purchase #7 $87.26   4.547%   $91.2277   $91.23   $91.25   $91.20  
  7 purchase totals = $905.40       $981.6098   $981.59   $981.60   $981.60