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 $246.57   4.636%   $258.001   $258.00   $258.00   $258.00  
  Purchase #2 $13.68   8.627%   $14.8602   $14.86   $14.85   $14.90  
  Purchase #3 $54.72   3.719%   $56.755   $56.76   $56.75   $56.80  
  Purchase #4 $176.56   13.824%   $200.9677   $200.97   $200.95   $201.00  
  Purchase #5 $166.88   12.291%   $187.3912   $187.39   $187.40   $187.40  
  Purchase #6 $83.98   5.568%   $88.656   $88.66   $88.65   $88.70  
  Purchase #7 $21.28   5.779%   $22.5098   $22.51   $22.50   $22.50  
  7 purchase totals = $763.67       $829.1409   $829.15   $829.10   $829.30