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 $133.71   4.245%   $139.386   $139.39   $139.40   $139.40  
  Purchase #2 $30.92   12.523%   $34.7921   $34.79   $34.80   $34.80  
  Purchase #3 $87.68   9.294%   $95.829   $95.83   $95.85   $95.80  
  Purchase #4 $149.48   8.859%   $162.7224   $162.72   $162.70   $162.70  
  Purchase #5 $109.60   11.141%   $121.8105   $121.81   $121.80   $121.80  
  Purchase #6 $39.49   9.158%   $43.1065   $43.11   $43.10   $43.10  
  Purchase #7 $92.64   9.706%   $101.6316   $101.63   $101.65   $101.60  
  7 purchase totals = $643.52       $699.2781   $699.28   $699.30   $699.20