IF you care about free Gems. You should do abyss until you get 33, the last 3 requires a lot more exp and skill, that people get with time. The reason you want to go there for 33, is just that until you get there, there is a lot of space to improve your acc. (comps, artifacts, skills...). So until you get to the 33 you should think about improving your acc. After being stable in 33* is like getting a free pass to do whatever since you will not be losing gems and potencial chars you want. 36* is still the main objective but i understand not everyone as the time and patience to get that level, if you didn't you also only lose 1200 gems a year... so not that important. If you can do 9 stars 11 floor you can do 6 stars 12 floor... most of the times is harder to do 11 stars 9 stars than 6 stars 12 floor because of 11-3.