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 $232.54   12.478%   $261.5563   $261.56   $261.55   $261.60  
  Purchase #2 $62.48   6.261%   $66.3919   $66.39   $66.40   $66.40  
  Purchase #3 $210.72   10.557%   $232.9657   $232.97   $232.95   $233.00  
  Purchase #4 $92.72   11.611%   $103.4857   $103.49   $103.50   $103.50  
  Purchase #5 $121.26   13.565%   $137.7089   $137.71   $137.70   $137.70  
  Purchase #6 $196.62   6.229%   $208.8675   $208.87   $208.85   $208.90  
  Purchase #7 $230.79   10.796%   $255.7061   $255.71   $255.70   $255.70  
  7 purchase totals = $1,147.13       $1,266.6821   $1,266.70   $1,266.65   $1,266.80