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 $23.46   12.438%   $26.378   $26.38   $26.40   $26.40  
  Purchase #2 $67.99   12.514%   $76.4983   $76.50   $76.50   $76.50  
  Purchase #3 $25.93   10.617%   $28.683   $28.68   $28.70   $28.70  
  Purchase #4 $58.99   9.437%   $64.5569   $64.56   $64.55   $64.60  
  Purchase #5 $59.66   9.801%   $65.5073   $65.51   $65.50   $65.50  
  Purchase #6 $80.81   5.637%   $85.3653   $85.37   $85.35   $85.40  
  Purchase #7 $231.90   3.777%   $240.6589   $240.66   $240.65   $240.70  
  7 purchase totals = $548.74       $587.6477   $587.66   $587.65   $587.80