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 $30.42   3.412%   $31.4579   $31.46   $31.45   $31.50  
  Purchase #2 $15.75   4.663%   $16.4844   $16.48   $16.50   $16.50  
  Purchase #3 $88.45   9.514%   $96.8651   $96.87   $96.85   $96.90  
  Purchase #4 $134.59   4.317%   $140.4003   $140.40   $140.40   $140.40  
  Purchase #5 $79.30   10.853%   $87.9064   $87.91   $87.90   $87.90  
  Purchase #6 $22.90   3.461%   $23.6926   $23.69   $23.70   $23.70  
  Purchase #7 $236.58   9.400%   $258.8185   $258.82   $258.80   $258.80  
  7 purchase totals = $607.99       $655.6252   $655.63   $655.60   $655.70