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 $229.64   6.866%   $245.4071   $245.41   $245.40   $245.40  
  Purchase #2 $115.27   3.213%   $118.9736   $118.97   $118.95   $119.00  
  Purchase #3 $140.28   7.353%   $150.5948   $150.59   $150.60   $150.60  
  Purchase #4 $248.31   3.346%   $256.6185   $256.62   $256.60   $256.60  
  Purchase #5 $199.75   3.471%   $206.6833   $206.68   $206.70   $206.70  
  Purchase #6 $80.98   3.118%   $83.505   $83.51   $83.50   $83.50  
  Purchase #7 $164.73   7.837%   $177.6399   $177.64   $177.65   $177.60  
  7 purchase totals = $1,178.96       $1,239.4222   $1,239.42   $1,239.40   $1,239.40