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 $126.72   10.799%   $140.4045   $140.40   $140.40   $140.40  
  Purchase #2 $220.42   10.481%   $243.5222   $243.52   $243.50   $243.50  
  Purchase #3 $9.57   7.349%   $10.2733   $10.27   $10.25   $10.30  
  Purchase #4 $75.52   6.514%   $80.4394   $80.44   $80.45   $80.40  
  Purchase #5 $83.80   3.858%   $87.033   $87.03   $87.05   $87.00  
  Purchase #6 $57.37   9.620%   $62.889   $62.89   $62.90   $62.90  
  Purchase #7 $14.71   7.582%   $15.8253   $15.83   $15.85   $15.80  
  7 purchase totals = $588.11       $640.3867   $640.38   $640.40   $640.30