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 $12.58   11.245%   $13.9946   $13.99   $14.00   $14.00  
  Purchase #2 $26.89   7.315%   $28.857   $28.86   $28.85   $28.90  
  Purchase #3 $239.81   8.807%   $260.9301   $260.93   $260.95   $260.90  
  Purchase #4 $122.50   6.504%   $130.4674   $130.47   $130.45   $130.50  
  Purchase #5 $130.85   7.512%   $140.6795   $140.68   $140.70   $140.70  
  Purchase #6 $29.59   9.287%   $32.338   $32.34   $32.35   $32.30  
  Purchase #7 $30.82   9.744%   $33.8231   $33.82   $33.80   $33.80  
  7 purchase totals = $593.04       $641.0897   $641.09   $641.10   $641.10