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 $137.66   3.624%   $142.6488   $142.65   $142.65   $142.60  
  Purchase #2 $15.81   11.863%   $17.6855   $17.69   $17.70   $17.70  
  Purchase #3 $88.60   9.107%   $96.6688   $96.67   $96.65   $96.70  
  Purchase #4 $7.74   11.311%   $8.6155   $8.62   $8.60   $8.60  
  Purchase #5 $161.55   11.516%   $180.1541   $180.15   $180.15   $180.20  
  Purchase #6 $71.57   6.314%   $76.0889   $76.09   $76.10   $76.10  
  Purchase #7 $17.90   5.424%   $18.8709   $18.87   $18.85   $18.90  
  7 purchase totals = $500.83       $540.7325   $540.74   $540.70   $540.80