Effective:
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)
Inept:
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.