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 $17.40   13.378%   $19.7278   $19.73   $19.75   $19.70  
  Purchase #2 $208.59   3.453%   $215.7926   $215.79   $215.80   $215.80  
  Purchase #3 $71.84   11.513%   $80.1109   $80.11   $80.10   $80.10  
  Purchase #4 $115.97   12.737%   $130.7411   $130.74   $130.75   $130.70  
  Purchase #5 $134.63   13.446%   $152.7323   $152.73   $152.75   $152.70  
  Purchase #6 $194.73   13.735%   $221.4762   $221.48   $221.50   $221.50  
  Purchase #7 $18.75   8.120%   $20.2725   $20.27   $20.25   $20.30  
  7 purchase totals = $761.91       $840.8534   $840.85   $840.90   $840.80