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 $28.94   6.834%   $30.9178   $30.92   $30.90   $30.90  
  Purchase #2 $107.83   12.550%   $121.3627   $121.36   $121.35   $121.40  
  Purchase #3 $143.87   3.304%   $148.6235   $148.62   $148.60   $148.60  
  Purchase #4 $25.39   4.670%   $26.5757   $26.58   $26.60   $26.60  
  Purchase #5 $223.53   7.120%   $239.4453   $239.45   $239.45   $239.40  
  Purchase #6 $41.31   12.357%   $46.4147   $46.41   $46.40   $46.40  
  Purchase #7 $26.31   8.516%   $28.5506   $28.55   $28.55   $28.60  
  7 purchase totals = $597.18       $641.8903   $641.89   $641.85   $641.90