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 $142.72   8.800%   $155.2794   $155.28   $155.30   $155.30  
  Purchase #2 $65.79   13.287%   $74.5315   $74.53   $74.55   $74.50  
  Purchase #3 $241.73   7.105%   $258.9049   $258.90   $258.90   $258.90  
  Purchase #4 $99.51   3.698%   $103.1899   $103.19   $103.20   $103.20  
  Purchase #5 $27.89   9.173%   $30.4483   $30.45   $30.45   $30.40  
  Purchase #6 $7.84   12.404%   $8.8125   $8.81   $8.80   $8.80  
  Purchase #7 $37.35   4.223%   $38.9273   $38.93   $38.95   $38.90  
  7 purchase totals = $622.83       $670.0938   $670.09   $670.15   $670.00