(I began with the first one and ended with the last one for a reason. I am entirely in favor of applause lights and positive-affect words as long as they serve their purpose -- community-building, making humans feel good -- and don't substitute for argument where the latter is appropriate.)
OK, a more accurate guide would be “Don't, unless you're discussing rationality itself (as opposed to an application thereof) or in other unusual circumstances.” But still.
Of that list the following the following make the kind of usage that the OP would target if it was not being flippantly general.
* Rational Romantic Relationships
* Rational Reading
* A Rational Education
* Rational Home Buying
The remainder use the word where it needs to be used, or at least provides information about the subject. With these examples the title need only convey the subject that is being analyzed or strategized about. We can assume (or hope) that the analysis and strategies provided are rational because they are on flipping lesswrong. The inclusion of "Rational" in the title is not necessarily a problem in all these cases but the general class of posts following this format does start to look ridiculous when used excessively or taken to extremes.
I notice that one of the examples is mine. I would use a different title were I to make that post today. I don't want to be associated with "Rational Toothpaste" vibes!
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The fact that the post itself is high-quality doesn't imply that it has the optimal title.
Why "Feeling Rational" has to have r-word in title; "Rational Romantic Relationships" would not lose much by changing to "Designing Better Romantic Relationships".