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 $12.74   3.548%   $13.192   $13.19   $13.20   $13.20  
  Purchase #2 $112.79   8.395%   $122.2587   $122.26   $122.25   $122.30  
  Purchase #3 $37.87   5.630%   $40.0021   $40.00   $40.00   $40.00  
  Purchase #4 $109.97   11.677%   $122.8112   $122.81   $122.80   $122.80  
  Purchase #5 $239.75   5.468%   $252.8595   $252.86   $252.85   $252.90  
  Purchase #6 $21.28   9.544%   $23.311   $23.31   $23.30   $23.30  
  Purchase #7 $86.93   12.340%   $97.6572   $97.66   $97.65   $97.70  
  7 purchase totals = $621.33       $672.0917   $672.09   $672.05   $672.20