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 $18.58   6.338%   $19.7576   $19.76   $19.75   $19.80  
  Purchase #2 $247.85   11.177%   $275.5522   $275.55   $275.55   $275.60  
  Purchase #3 $72.83   13.678%   $82.7917   $82.79   $82.80   $82.80  
  Purchase #4 $104.99   12.392%   $118.0004   $118.00   $118.00   $118.00  
  Purchase #5 $226.90   4.117%   $236.2415   $236.24   $236.25   $236.20  
  Purchase #6 $224.84   11.166%   $249.9456   $249.95   $249.95   $249.90  
  Purchase #7 $186.28   5.546%   $196.6111   $196.61   $196.60   $196.60  
  7 purchase totals = $1,082.27       $1,178.9001   $1,178.90   $1,178.90   $1,178.90