(i) If for though. If should not be used in the sense of though; it causes ambiguity.
The letters that come from that department are well-reasoned if long-winded.
Does if here mean if or though? In the one case the sentence will mean that the long-windedness of the letters makes them well-reasoned; in the other case it will mean that they are well reasoned in spite of their long-windedness. There is no excuse for writing an ambiguous sentence when it might have been written as easily without ambiguity. Those who get into the habit of using if for though may land themselves in such absurdities as the example given by Sir Alan Herbert:
Milk is nourishing, if tuberculous. ( Words and Idioms. Constable & Co.)
(ii) If for whether
The use of if for whether is also a fruitful cause of ambiguity. A form I have to fill up in order to get something I want contains the entry:
State here if cultivation contract work is undertaken.
Does this mean "state here whether cultivation contract work is undertaken, yes or no", or does it mean "if cultivation contract work is undertaken, this is the place to say so, but if it is not you need say nothing about the subject either here or anywhere else"? Unnecessary ambiguity should always be avoided, and ambiguity caused by using if instead of whether is wantonly unnecessary.