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 $16.91   12.308%   $18.9913   $18.99   $19.00   $19.00  
  Purchase #2 $53.74   10.577%   $59.4241   $59.42   $59.40   $59.40  
  Purchase #3 $47.50   8.373%   $51.4772   $51.48   $51.50   $51.50  
  Purchase #4 $123.62   9.330%   $135.1537   $135.15   $135.15   $135.20  
  Purchase #5 $174.83   6.238%   $185.7359   $185.74   $185.75   $185.70  
  Purchase #6 $33.82   5.725%   $35.7562   $35.76   $35.75   $35.80  
  Purchase #7 $92.68   9.744%   $101.7107   $101.71   $101.70   $101.70  
  7 purchase totals = $543.10       $588.2491   $588.25   $588.25   $588.30