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 $243.72   6.258%   $258.972   $258.97   $258.95   $259.00  
  Purchase #2 $42.53   13.390%   $48.2248   $48.22   $48.20   $48.20  
  Purchase #3 $22.60   6.119%   $23.9829   $23.98   $24.00   $24.00  
  Purchase #4 $73.91   6.605%   $78.7918   $78.79   $78.80   $78.80  
  Purchase #5 $199.69   7.419%   $214.505   $214.51   $214.50   $214.50  
  Purchase #6 $81.74   7.311%   $87.716   $87.72   $87.70   $87.70  
  Purchase #7 $125.55   13.411%   $142.3875   $142.39   $142.40   $142.40  
  7 purchase totals = $789.74       $854.5800   $854.58   $854.55   $854.60