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 $5.95   12.647%   $6.7025   $6.70   $6.70   $6.70  
  Purchase #2 $63.26   13.432%   $71.7571   $71.76   $71.75   $71.80  
  Purchase #3 $52.62   7.517%   $56.5754   $56.58   $56.60   $56.60  
  Purchase #4 $106.43   7.296%   $114.1951   $114.20   $114.20   $114.20  
  Purchase #5 $232.65   12.208%   $261.0519   $261.05   $261.05   $261.10  
  Purchase #6 $239.63   8.443%   $259.862   $259.86   $259.85   $259.90  
  Purchase #7 $223.45   12.365%   $251.0796   $251.08   $251.10   $251.10  
  7 purchase totals = $923.99       $1,021.2236   $1,021.23   $1,021.25   $1,021.40