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 $234.70   8.522%   $254.7011   $254.70   $254.70   $254.70  
  Purchase #2 $28.44   9.477%   $31.1353   $31.14   $31.15   $31.10  
  Purchase #3 $15.32   12.142%   $17.1802   $17.18   $17.20   $17.20  
  Purchase #4 $76.53   13.509%   $86.8684   $86.87   $86.85   $86.90  
  Purchase #5 $145.49   12.824%   $164.1476   $164.15   $164.15   $164.10  
  Purchase #6 $69.99   6.796%   $74.7465   $74.75   $74.75   $74.70  
  Purchase #7 $222.76   11.651%   $248.7138   $248.71   $248.70   $248.70  
  7 purchase totals = $793.23       $877.4929   $877.50   $877.50   $877.40