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 $16.99   4.833%   $17.8111   $17.81   $17.80   $17.80  
  Purchase #2 $7.84   8.873%   $8.5356   $8.54   $8.55   $8.50  
  Purchase #3 $232.36   11.534%   $259.1604   $259.16   $259.15   $259.20  
  Purchase #4 $173.48   7.702%   $186.8414   $186.84   $186.85   $186.80  
  Purchase #5 $17.63   3.698%   $18.282   $18.28   $18.30   $18.30  
  Purchase #6 $99.92   10.704%   $110.6154   $110.62   $110.60   $110.60  
  Purchase #7 $53.50   13.659%   $60.8076   $60.81   $60.80   $60.80  
  7 purchase totals = $601.72       $662.0535   $662.06   $662.05   $662.00