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 $58.34   12.463%   $65.6109   $65.61   $65.60   $65.60  
  Purchase #2 $88.98   4.644%   $93.1122   $93.11   $93.10   $93.10  
  Purchase #3 $163.69   12.174%   $183.6176   $183.62   $183.60   $183.60  
  Purchase #4 $100.99   13.533%   $114.657   $114.66   $114.65   $114.70  
  Purchase #5 $69.49   7.151%   $74.4592   $74.46   $74.45   $74.50  
  Purchase #6 $18.67   11.192%   $20.7595   $20.76   $20.75   $20.80  
  Purchase #7 $24.63   9.695%   $27.0179   $27.02   $27.00   $27.00  
  7 purchase totals = $524.79       $579.2343   $579.24   $579.15   $579.30