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 $38.67   3.435%   $39.9983   $40.00   $40.00   $40.00  
  Purchase #2 $193.82   5.737%   $204.9395   $204.94   $204.95   $204.90  
  Purchase #3 $18.75   9.845%   $20.5959   $20.60   $20.60   $20.60  
  Purchase #4 $16.65   13.219%   $18.851   $18.85   $18.85   $18.90  
  Purchase #5 $98.44   13.447%   $111.6772   $111.68   $111.70   $111.70  
  Purchase #6 $138.40   9.857%   $152.0421   $152.04   $152.05   $152.00  
  Purchase #7 $215.53   7.792%   $232.3241   $232.32   $232.30   $232.30  
  7 purchase totals = $720.26       $780.4281   $780.43   $780.45   $780.40