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 $218.78   13.156%   $247.5627   $247.56   $247.55   $247.60  
  Purchase #2 $93.95   12.417%   $105.6158   $105.62   $105.60   $105.60  
  Purchase #3 $115.39   12.208%   $129.4768   $129.48   $129.50   $129.50  
  Purchase #4 $70.60   5.393%   $74.4075   $74.41   $74.40   $74.40  
  Purchase #5 $23.69   12.586%   $26.6716   $26.67   $26.65   $26.70  
  Purchase #6 $40.78   5.869%   $43.1734   $43.17   $43.15   $43.20  
  Purchase #7 $209.88   5.371%   $221.1527   $221.15   $221.15   $221.20  
  7 purchase totals = $773.07       $848.0605   $848.06   $848.00   $848.20