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 $16.54   9.419%   $18.0979   $18.10   $18.10   $18.10  
  Purchase #2 $21.34   8.841%   $23.2267   $23.23   $23.25   $23.20  
  Purchase #3 $17.47   8.337%   $18.9265   $18.93   $18.95   $18.90  
  Purchase #4 $114.90   13.569%   $130.4908   $130.49   $130.50   $130.50  
  Purchase #5 $63.97   10.699%   $70.8142   $70.81   $70.80   $70.80  
  Purchase #6 $173.46   5.853%   $183.6126   $183.61   $183.60   $183.60  
  Purchase #7 $23.49   6.428%   $24.9999   $25.00   $25.00   $25.00  
  7 purchase totals = $431.17       $470.1686   $470.17   $470.20   $470.10