You do benefit from learning joseki and that learning “by trial and error” is woefully inefficient. If you know that you’ve played strongly (locally) in the corners, then you can turn your focus of improvement onto the other aspects of the game.
What I want to make clear, though, is that you shouldn’t actually sit down and learn a lot of joseki at once, at least whilst you’re below 5k. If you’re a beginner then I’d say learn these following joseki whilst you play, looking at them one at a time every now and again. If you’ve already learn these (and you’re less than 5k), then every time you mess up in a corner and get a bad result you should first try to work out where you went wrong on your own, and then look up where you deviated from joseki and what you should have played.
These are, in my opinion, the basic joseki for a 9k player. Doesn’t mean that you shouldn’t learn others if you want to but these, I think, would be the baseline knowledge that you’d require for that rank (I personally am 7 – 8k.).
- 4-4, approach, keima away, slide http://josekipedia.com/#path:pdqfncrdqcqi
- 4-4, approach, keima away, ogeima away http://josekipedia.com/#path:pdqfncpi
- 4-4, approach, pincer (to face towards pincer stone) http://josekipedia.com/#path:pdqfqhqcqdpcodrdrercqenc
- 4-4, approach, pincer (to face away from pincer stone) http://josekipedia.com/#path:pdqfqhqcpcqdperfog
- 4-4, approach, kick http://josekipedia.com/#path:pdqfqepfndqj
- Classic 3-3 invasion http://josekipedia.com/#path:pdqcqdpcocobncnbmcrdrercqf
- 3-3 invasion, double hane http://josekipedia.com/#path:pdqcqdpcocobnbncodmbpbnaqb
- 3-4, high approach, full triangle and extension http://josekipedia.com/#path:qdodocncpcndqfjd
- 3-4, high approach, pincer http://josekipedia.com/#path:qdodmdqcrcpcreof
- 3-4, low approach, Shusaku kosumi http://josekipedia.com/#path:qdocpe
- 3-4, low approach, pincer and cut (admittedly, I didn’t actually know this one at 9k as I would attach underneath instead) http://josekipedia.com/#path:qdocmcpepdodoeneofmdpflc
- 5-3, low approach, press (also a 3-4 tenuki joseki) http://josekipedia.com/#path:qepcodocndmc
- 5-4, 3-4 invasion (also a 3-4 tenuki joseki), inside contact http://josekipedia.com/#path:pepcqcqbqdobqh