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 $218.95   13.817%   $249.2023   $249.20   $249.20   $249.20  
  Purchase #2 $95.86   9.675%   $105.1345   $105.13   $105.15   $105.10  
  Purchase #3 $92.80   4.338%   $96.8257   $96.83   $96.85   $96.80  
  Purchase #4 $198.74   6.863%   $212.3795   $212.38   $212.40   $212.40  
  Purchase #5 $56.83   11.738%   $63.5007   $63.50   $63.50   $63.50  
  Purchase #6 $177.59   3.256%   $183.3723   $183.37   $183.35   $183.40  
  Purchase #7 $75.43   7.674%   $81.2185   $81.22   $81.20   $81.20  
  7 purchase totals = $916.20       $991.6335   $991.63   $991.65   $991.60