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 $20.42   6.654%   $21.7787   $21.78   $21.80   $21.80  
  Purchase #2 $51.27   8.370%   $55.5613   $55.56   $55.55   $55.60  
  Purchase #3 $196.97   10.483%   $217.6184   $217.62   $217.60   $217.60  
  Purchase #4 $74.69   8.624%   $81.1313   $81.13   $81.15   $81.10  
  Purchase #5 $62.47   10.264%   $68.8819   $68.88   $68.90   $68.90  
  Purchase #6 $10.85   12.855%   $12.2448   $12.24   $12.25   $12.20  
  Purchase #7 $154.89   13.301%   $175.4919   $175.49   $175.50   $175.50  
  7 purchase totals = $571.56       $632.7083   $632.70   $632.75   $632.70