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 $27.69   11.551%   $30.8885   $30.89   $30.90   $30.90  
  Purchase #2 $120.87   12.846%   $136.397   $136.40   $136.40   $136.40  
  Purchase #3 $128.99   13.263%   $146.0979   $146.10   $146.10   $146.10  
  Purchase #4 $173.87   12.235%   $195.143   $195.14   $195.15   $195.10  
  Purchase #5 $176.25   7.224%   $188.9823   $188.98   $189.00   $189.00  
  Purchase #6 $53.55   8.804%   $58.2645   $58.26   $58.25   $58.30  
  Purchase #7 $52.44   7.516%   $56.3814   $56.38   $56.40   $56.40  
  7 purchase totals = $733.66       $812.1546   $812.15   $812.20   $812.20