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 $229.44   3.872%   $238.3239   $238.32   $238.30   $238.30  
  Purchase #2 $11.37   7.141%   $12.1819   $12.18   $12.20   $12.20  
  Purchase #3 $72.32   3.134%   $74.5865   $74.59   $74.60   $74.60  
  Purchase #4 $226.42   8.749%   $246.2295   $246.23   $246.25   $246.20  
  Purchase #5 $164.73   6.338%   $175.1706   $175.17   $175.15   $175.20  
  Purchase #6 $27.49   10.152%   $30.2808   $30.28   $30.30   $30.30  
  Purchase #7 $208.38   12.850%   $235.1568   $235.16   $235.15   $235.20  
  7 purchase totals = $940.15       $1,011.9300   $1,011.93   $1,011.95   $1,012.00