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 $28.73   4.365%   $29.9841   $29.98   $30.00   $30.00  
  Purchase #2 $216.43   5.357%   $228.0242   $228.02   $228.00   $228.00  
  Purchase #3 $169.68   10.361%   $187.2605   $187.26   $187.25   $187.30  
  Purchase #4 $113.76   13.441%   $129.0505   $129.05   $129.05   $129.10  
  Purchase #5 $234.36   3.529%   $242.6306   $242.63   $242.65   $242.60  
  Purchase #6 $64.67   7.426%   $69.4724   $69.47   $69.45   $69.50  
  Purchase #7 $84.59   11.690%   $94.4786   $94.48   $94.50   $94.50  
  7 purchase totals = $912.22       $980.9009   $980.89   $980.90   $981.00