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 $28.30   7.333%   $30.3752   $30.38   $30.40   $30.40  
  Purchase #2 $137.27   6.702%   $146.4698   $146.47   $146.45   $146.50  
  Purchase #3 $38.45   11.216%   $42.7626   $42.76   $42.75   $42.80  
  Purchase #4 $214.76   10.535%   $237.385   $237.39   $237.40   $237.40  
  Purchase #5 $143.31   4.239%   $149.3849   $149.38   $149.40   $149.40  
  Purchase #6 $194.33   5.172%   $204.3807   $204.38   $204.40   $204.40  
  Purchase #7 $9.38   7.729%   $10.105   $10.11   $10.10   $10.10  
  7 purchase totals = $765.80       $820.8632   $820.87   $820.90   $821.00