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 $219.80   13.725%   $249.9676   $249.97   $249.95   $250.00  
  Purchase #2 $236.67   4.264%   $246.7616   $246.76   $246.75   $246.80  
  Purchase #3 $31.59   10.279%   $34.8371   $34.84   $34.85   $34.80  
  Purchase #4 $180.39   8.227%   $195.2307   $195.23   $195.25   $195.20  
  Purchase #5 $139.80   13.382%   $158.508   $158.51   $158.50   $158.50  
  Purchase #6 $86.66   10.307%   $95.592   $95.59   $95.60   $95.60  
  Purchase #7 $40.50   4.756%   $42.4262   $42.43   $42.45   $42.40  
  7 purchase totals = $935.41       $1,023.3232   $1,023.33   $1,023.35   $1,023.30