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 $19.95   3.583%   $20.6648   $20.66   $20.65   $20.70  
  Purchase #2 $107.64   9.534%   $117.9024   $117.90   $117.90   $117.90  
  Purchase #3 $244.59   5.360%   $257.7   $257.70   $257.70   $257.70  
  Purchase #4 $113.35   13.871%   $129.0728   $129.07   $129.05   $129.10  
  Purchase #5 $25.86   12.627%   $29.1253   $29.13   $29.15   $29.10  
  Purchase #6 $30.49   4.715%   $31.9276   $31.93   $31.95   $31.90  
  Purchase #7 $236.66   6.532%   $252.1186   $252.12   $252.10   $252.10  
  7 purchase totals = $778.54       $838.5115   $838.51   $838.50   $838.50