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 $241.78   6.745%   $258.0881   $258.09   $258.10   $258.10  
  Purchase #2 $29.92   13.382%   $33.9239   $33.92   $33.90   $33.90  
  Purchase #3 $59.42   5.623%   $62.7612   $62.76   $62.75   $62.80  
  Purchase #4 $213.61   8.545%   $231.863   $231.86   $231.85   $231.90  
  Purchase #5 $5.91   6.299%   $6.2823   $6.28   $6.30   $6.30  
  Purchase #6 $106.44   12.336%   $119.5704   $119.57   $119.55   $119.60  
  Purchase #7 $86.37   9.201%   $94.3169   $94.32   $94.30   $94.30  
  7 purchase totals = $743.45       $806.8058   $806.80   $806.75   $806.90