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 $223.45   13.460%   $253.5264   $253.53   $253.55   $253.50  
  Purchase #2 $123.97   13.441%   $140.6328   $140.63   $140.65   $140.60  
  Purchase #3 $92.86   6.357%   $98.7631   $98.76   $98.75   $98.80  
  Purchase #4 $61.59   13.548%   $69.9342   $69.93   $69.95   $69.90  
  Purchase #5 $110.66   10.318%   $122.0779   $122.08   $122.10   $122.10  
  Purchase #6 $158.44   11.871%   $177.2484   $177.25   $177.25   $177.20  
  Purchase #7 $152.86   3.856%   $158.7543   $158.75   $158.75   $158.80  
  7 purchase totals = $923.83       $1,020.9371   $1,020.93   $1,021.00   $1,020.90