Word of God
The motto is deliberately practical advice.
Ailsa Floyd for the Times Educational Supplement in Scotland - How did you think up the motto "Never tickle sleeping dragons", which appears under the crest? Is there a story about it?
JK Rowling: You know the way that most school slogans are thing like persevere and nobility, charity and fidelity or something, it just amused me to give an entirely practical piece of advice for the Hogwarts school motto.
Then a friend of mine who is a professor of classics - my Latin was not up to the job, I did not think it should be cod Latin, it is good enough for cod Latin spells, that is they used to be a mixture of Latin and other things. When it came to a proper Latin slogan for the school I wanted it to be right, I went to him and asked him to translate. I think he really enjoyed it, he rang me up and said, "I think I found the exactly right word, 'Titillandus'", that was how that was dreamt up.
(source: Edinburgh "cub reporter" press conference, ITV, 16 July 2005)
Is it an "obvious advice"?
Not really, since it's an advice regularly disregarded by many students.
Consider every single thing Harry did (tackling, in some semblence of order: a fully grown mountain troll, Fluffy, Voldemort, Aragog and his offspring, Whomping Willow, basilisk, Tom Riddle's diary, Sirius Black, a gazillion Dementors, a dragon, mermaids and Grindylows, monsters in a maze, Voldemort again... and I'm only up to book 4 and going from memory. Bold ones he tackled voluntarily).
Now consider every single thing the Marauders did.
Still think it's trivial, obvious, useless advice, either literally or metaphorically? :)
Bonus pedantry: technically, Latin translation is "a sleeping dragon must never be tickled", though the difference isn't semantically meaningful.