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 $28.55   6.563%   $30.4237   $30.42   $30.40   $30.40  
  Purchase #2 $137.71   4.646%   $144.108   $144.11   $144.10   $144.10  
  Purchase #3 $228.56   10.220%   $251.9188   $251.92   $251.90   $251.90  
  Purchase #4 $39.29   5.418%   $41.4187   $41.42   $41.40   $41.40  
  Purchase #5 $19.30   6.785%   $20.6095   $20.61   $20.60   $20.60  
  Purchase #6 $218.80   7.383%   $234.954   $234.95   $234.95   $235.00  
  Purchase #7 $17.64   5.647%   $18.6361   $18.64   $18.65   $18.60  
  7 purchase totals = $689.85       $742.0688   $742.07   $742.00   $742.00