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 $19.26   8.478%   $20.8929   $20.89   $20.90   $20.90  
  Purchase #2 $30.71   4.611%   $32.126   $32.13   $32.15   $32.10  
  Purchase #3 $248.57   11.818%   $277.946   $277.95   $277.95   $277.90  
  Purchase #4 $249.79   4.504%   $261.0405   $261.04   $261.05   $261.00  
  Purchase #5 $29.66   13.476%   $33.657   $33.66   $33.65   $33.70  
  Purchase #6 $37.42   7.214%   $40.1195   $40.12   $40.10   $40.10  
  Purchase #7 $125.70   4.529%   $131.393   $131.39   $131.40   $131.40  
  7 purchase totals = $741.11       $797.1749   $797.18   $797.20   $797.10