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 $37.27   13.368%   $42.2523   $42.25   $42.25   $42.30  
  Purchase #2 $55.84   7.466%   $60.009   $60.01   $60.00   $60.00  
  Purchase #3 $140.50   7.814%   $151.4787   $151.48   $151.50   $151.50  
  Purchase #4 $14.93   4.334%   $15.5771   $15.58   $15.60   $15.60  
  Purchase #5 $218.56   8.134%   $236.3377   $236.34   $236.35   $236.30  
  Purchase #6 $31.90   7.774%   $34.3799   $34.38   $34.40   $34.40  
  Purchase #7 $142.71   3.687%   $147.9717   $147.97   $147.95   $148.00  
  7 purchase totals = $641.71       $688.0064   $688.01   $688.05   $688.10