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 $54.52   10.630%   $60.3155   $60.32   $60.30   $60.30  
  Purchase #2 $47.81   3.295%   $49.3853   $49.39   $49.40   $49.40  
  Purchase #3 $50.62   10.447%   $55.9083   $55.91   $55.90   $55.90  
  Purchase #4 $119.64   6.110%   $126.95   $126.95   $126.95   $127.00  
  Purchase #5 $57.44   10.217%   $63.3086   $63.31   $63.30   $63.30  
  Purchase #6 $242.44   3.756%   $251.546   $251.55   $251.55   $251.50  
  Purchase #7 $59.69   7.371%   $64.0897   $64.09   $64.10   $64.10  
  7 purchase totals = $632.16       $671.5034   $671.52   $671.50   $671.50