There's no hard and fast rule, but many people use quotation marks for technical or unfamiliar words used for the first time (and defined either explicitly or in context); also, for highlighted or colloquial terms, which in these cases some sources might similarly recommend only putting in quotation marks the first time while others may allow that they should be in quotation marks every time used.
A Purdue University Online Writing Lab document (http://owl.english.purdue.edu/) republished by the University or Oregon has these instructions:
"Use quotation marks to indicate words used ironically, with
reservations, or in some unusual way.....For words used as words
themselves or for technical or unfamiliar terms used for the first
time (and defined), use italics."
While this source recommends to use italics many other sources suggest quotation marks, and especially when writing by hand italics can be difficult to use so impractical.
A University of Melbourne document has these related instructions:
"examples of how quotation marks are used in different situations.....
- Use single quotation marks to indicate that a word or phrase is
used in a special way or that the word is the topic under discussion:
- Use quotation marks the first time a technical term is used in a document for a general audience:
- Use quotation marks to highlight a questionable concept:
- Use quotation marks for colloquial words in academic writing:"
Here is the guidance from the American Psychological Association for professional papers:
"Use double quotation marks ... to introduce a word or phrase used as
an ironic comment, as slang, or as an invented or coined expression.
Use quotation marks the first time the word or phrase is used;
thereafter, do not use quotation marks."
So what I am hoping to show from these examples is:
1) Credible sources often have similar but slightly different instructions
2) Generally sources prefer that a defined term is only put in quotation marks the first time, and afterwards not put in quotation marks
3) But for some terms some sources will allow using quotation marks every time.
Just to give a short example where it might be appropriate to use quotation marks every time a word is used.
Example: Some people use the team "bad" to mean good. A "bad ass", for instance, can mean the most accomplished person at a particular skill set. But the question arises, if these people use an altered language where "bad" means good, does this language become an impediment to recognizing what is traditionally considered bad and good? Do "bad"-speaking people make bad decisions because they poorly recognize potential decision trees which will likely have an unfavorable outcome?
Another example: I was told very young in life that it was "improper" for me to attempt to improve my social status or to mingle with those of a higher social status. I wonder what kind of proper rules of society can exist when what is considered "improper" eliminates one of the very best qualities of humanity, the ability to improve ourselves? And on a broader scale, I would think that such a definition of "improper" is quite incorrect if one hopes to have a successful society free of social upheaval.
But despite these cases where it may be appropriate to use quotation marks each time, in general my understanding is that they should normally only be used the first time a term requiring them is used, and cases where quotation marks are used more than just the first time for a term should be exceptions to the general rule for clarity or emphasis.