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 $23.48   13.230%   $26.5864   $26.59   $26.60   $26.60  
  Purchase #2 $42.86   10.581%   $47.395   $47.40   $47.40   $47.40  
  Purchase #3 $45.58   4.706%   $47.725   $47.73   $47.75   $47.70  
  Purchase #4 $32.83   7.113%   $35.1652   $35.17   $35.15   $35.20  
  Purchase #5 $71.52   12.587%   $80.5222   $80.52   $80.50   $80.50  
  Purchase #6 $53.99   8.618%   $58.6429   $58.64   $58.65   $58.60  
  Purchase #7 $41.97   4.320%   $43.7831   $43.78   $43.80   $43.80  
  7 purchase totals = $312.23       $339.8198   $339.83   $339.85   $339.80