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 $63.94   4.564%   $66.8582   $66.86   $66.85   $66.90  
  Purchase #2 $10.76   10.517%   $11.8916   $11.89   $11.90   $11.90  
  Purchase #3 $179.99   13.686%   $204.6234   $204.62   $204.60   $204.60  
  Purchase #4 $234.68   13.132%   $265.4982   $265.50   $265.50   $265.50  
  Purchase #5 $16.65   5.765%   $17.6099   $17.61   $17.60   $17.60  
  Purchase #6 $29.80   6.352%   $31.6929   $31.69   $31.70   $31.70  
  Purchase #7 $74.65   12.663%   $84.1029   $84.10   $84.10   $84.10  
  7 purchase totals = $610.47       $682.2771   $682.27   $682.25   $682.30