I imagine you might get different opinions on this, but having used Otto and dagger myself, and considering the information you provided, I don't think the cost/benefit of implementing DI (with dagger, guice, or something else) would be worth it for you. This is not a knock on dagger - I like it and am using it successfully, but there's a learning curve, and if you're not doing unit testing, I don't know that it would provide much value to you.
If you begin to do unit testing later, then adding DI would be a good move.
Regarding the question of separate Bus
instances for separate activity-fragment groups, that may not be necessary if those activities-fragment groups are not publishing and subscribing to the same kinds of events. That is, if activity-fragment group A only cares about Foo events, and activity-fragment group B only cares about Bar events, then I think a single Application-scope Bus
might meet your needs.