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 $138.49   11.738%   $154.746   $154.75   $154.75   $154.70  
  Purchase #2 $95.27   13.562%   $108.1905   $108.19   $108.20   $108.20  
  Purchase #3 $32.83   12.863%   $37.0529   $37.05   $37.05   $37.10  
  Purchase #4 $141.53   10.381%   $156.2222   $156.22   $156.20   $156.20  
  Purchase #5 $67.91   8.823%   $73.9017   $73.90   $73.90   $73.90  
  Purchase #6 $63.68   8.229%   $68.9202   $68.92   $68.90   $68.90  
  Purchase #7 $241.38   3.567%   $249.99   $249.99   $250.00   $250.00  
  7 purchase totals = $781.09       $849.0235   $849.02   $849.00   $849.00