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 $29.57   4.176%   $30.8048   $30.80   $30.80   $30.80  
  Purchase #2 $93.81   7.847%   $101.1713   $101.17   $101.15   $101.20  
  Purchase #3 $11.72   8.410%   $12.7057   $12.71   $12.70   $12.70  
  Purchase #4 $93.41   11.700%   $104.339   $104.34   $104.35   $104.30  
  Purchase #5 $143.40   7.155%   $153.6603   $153.66   $153.65   $153.70  
  Purchase #6 $47.75   5.622%   $50.4345   $50.43   $50.45   $50.40  
  Purchase #7 $134.28   7.856%   $144.829   $144.83   $144.85   $144.80  
  7 purchase totals = $553.94       $597.9446   $597.94   $597.95   $597.90