The vast majority of us do, Chips. Especially those with Crohn's involving the ileum, it's really the only spot that processes B12, and the area gets so ravaged from Crohn's that even when not flaring, we just don't absorb as normal.
That depends on a variety of variables. The process by which B12 goes from food to being absorbed is very complex and Crohn's Disease can interfere with every single one. If the reason for your B12 deficiency is due to active Crohn's Disease and no longer affects you when in remission, then you might not need to always supplement B12. In the end, regular tests of your B12 levels will let you know. If you can maintain levels above 500 pg/ml (in my opinion the low end of the reference range is way too low and I'm happy to explain why) without supplementation then great. But dropping below that is playing with fire and in my opinion supplements should be taken to ensure that does not happen.
I agree with david. its better not to take any kind of risk...having higher level of B12 is always beter than any kind of risk of deficiency...and whats best is that even if you have very high level of B12 , there is no danger of of any harmful effect...its always better to have adequate levels than any kind of risks...B12 supplements should not only be important for crohns sufferers but also any normal person.