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 $17.80   12.874%   $20.0916   $20.09   $20.10   $20.10  
  Purchase #2 $220.26   8.745%   $239.5217   $239.52   $239.50   $239.50  
  Purchase #3 $22.58   4.755%   $23.6537   $23.65   $23.65   $23.70  
  Purchase #4 $70.25   4.734%   $73.5756   $73.58   $73.60   $73.60  
  Purchase #5 $31.40   9.509%   $34.3858   $34.39   $34.40   $34.40  
  Purchase #6 $20.67   4.749%   $21.6516   $21.65   $21.65   $21.70  
  Purchase #7 $242.35   13.670%   $275.4792   $275.48   $275.50   $275.50  
  7 purchase totals = $625.31       $688.3592   $688.36   $688.40   $688.50