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 $17.48   13.333%   $19.8106   $19.81   $19.80   $19.80  
  Purchase #2 $10.90   5.560%   $11.506   $11.51   $11.50   $11.50  
  Purchase #3 $172.69   3.118%   $178.0745   $178.07   $178.05   $178.10  
  Purchase #4 $154.89   3.737%   $160.6782   $160.68   $160.70   $160.70  
  Purchase #5 $74.75   8.473%   $81.0836   $81.08   $81.10   $81.10  
  Purchase #6 $54.72   7.519%   $58.8344   $58.83   $58.85   $58.80  
  Purchase #7 $48.27   3.125%   $49.7784   $49.78   $49.80   $49.80  
  7 purchase totals = $533.70       $559.7657   $559.76   $559.80   $559.80