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 $39.70   7.749%   $42.7764   $42.78   $42.80   $42.80  
  Purchase #2 $143.55   8.544%   $155.8149   $155.81   $155.80   $155.80  
  Purchase #3 $9.32   4.824%   $9.7696   $9.77   $9.75   $9.80  
  Purchase #4 $201.71   7.831%   $217.5059   $217.51   $217.50   $217.50  
  Purchase #5 $62.59   13.699%   $71.1642   $71.16   $71.15   $71.20  
  Purchase #6 $37.92   12.383%   $42.6156   $42.62   $42.60   $42.60  
  Purchase #7 $122.99   5.643%   $129.9303   $129.93   $129.95   $129.90  
  7 purchase totals = $617.78       $669.5769   $669.58   $669.55   $669.60