Speed [of walking] is limited not by muscle power but by the length of the legs. . . . This is why little children have to break into a run to keep up with their parents, and why people on crutches can walk surprisingly fast--their "legs" extend from their armpits to the ground. (Ingram 93)
You may have seen this kind of sign at the supermarket. While seeking emphasis, the sign proclaims that the fish is not really fresh.
In most cases a well-constructed sentence can make irony clear without shouting--which is the effect of putting a word in quotation marks. Similarly, the use of quotes to indicate a word that is considered slang is counterproductive--it draws attention to the fact that you have been unable to find an appropriate word, rather than making the slang acceptable.