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 $227.87   13.682%   $259.0472   $259.05   $259.05   $259.00  
  Purchase #2 $108.92   6.181%   $115.6523   $115.65   $115.65   $115.70  
  Purchase #3 $89.97   12.472%   $101.1911   $101.19   $101.20   $101.20  
  Purchase #4 $147.39   12.235%   $165.4232   $165.42   $165.40   $165.40  
  Purchase #5 $183.27   8.445%   $198.7472   $198.75   $198.75   $198.70  
  Purchase #6 $245.28   11.306%   $273.0114   $273.01   $273.00   $273.00  
  Purchase #7 $216.29   4.546%   $226.1225   $226.12   $226.10   $226.10  
  7 purchase totals = $1,218.99       $1,339.1949   $1,339.19   $1,339.15   $1,339.10