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 $128.44   8.257%   $139.0453   $139.05   $139.05   $139.00  
  Purchase #2 $7.27   5.325%   $7.6571   $7.66   $7.65   $7.70  
  Purchase #3 $30.96   12.120%   $34.7124   $34.71   $34.70   $34.70  
  Purchase #4 $169.86   12.219%   $190.6152   $190.62   $190.60   $190.60  
  Purchase #5 $56.93   9.768%   $62.4909   $62.49   $62.50   $62.50  
  Purchase #6 $130.57   9.160%   $142.5302   $142.53   $142.55   $142.50  
  Purchase #7 $84.59   5.622%   $89.3456   $89.35   $89.35   $89.30  
  7 purchase totals = $608.62       $666.3967   $666.41   $666.40   $666.30