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 $129.92   12.562%   $146.2406   $146.24   $146.25   $146.20  
  Purchase #2 $107.93   13.539%   $122.5426   $122.54   $122.55   $122.50  
  Purchase #3 $234.78   13.476%   $266.419   $266.42   $266.40   $266.40  
  Purchase #4 $81.99   9.714%   $89.9545   $89.95   $89.95   $90.00  
  Purchase #5 $57.47   3.287%   $59.359   $59.36   $59.35   $59.40  
  Purchase #6 $162.55   12.605%   $183.0394   $183.04   $183.05   $183.00  
  Purchase #7 $48.26   10.395%   $53.2766   $53.28   $53.30   $53.30  
  7 purchase totals = $822.90       $920.8317   $920.83   $920.85   $920.80