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 $231.65   11.822%   $259.0357   $259.04   $259.05   $259.00  
  Purchase #2 $217.97   13.867%   $248.1959   $248.20   $248.20   $248.20  
  Purchase #3 $63.40   9.563%   $69.4629   $69.46   $69.45   $69.50  
  Purchase #4 $62.66   8.285%   $67.8514   $67.85   $67.85   $67.90  
  Purchase #5 $183.62   13.296%   $208.0341   $208.03   $208.05   $208.00  
  Purchase #6 $223.32   7.129%   $239.2405   $239.24   $239.25   $239.20  
  Purchase #7 $82.26   3.495%   $85.135   $85.14   $85.15   $85.10  
  7 purchase totals = $1,064.88       $1,176.9555   $1,176.96   $1,177.00   $1,176.90