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 $229.73   12.873%   $259.3031   $259.30   $259.30   $259.30  
  Purchase #2 $15.73   3.197%   $16.2329   $16.23   $16.25   $16.20  
  Purchase #3 $61.30   3.589%   $63.5001   $63.50   $63.50   $63.50  
  Purchase #4 $204.69   6.229%   $217.4401   $217.44   $217.45   $217.40  
  Purchase #5 $124.81   13.733%   $141.9502   $141.95   $141.95   $142.00  
  Purchase #6 $56.96   13.220%   $64.4901   $64.49   $64.50   $64.50  
  Purchase #7 $40.49   5.832%   $42.8514   $42.85   $42.85   $42.90  
  7 purchase totals = $733.71       $805.7679   $805.76   $805.80   $805.80