Re: Is a tsuba necessary?
Depends on what you're doing. First, ask your teacher and do what he or she says, if there's no rule or requirement at that level, look at what the senior students do and copy them.
In many aikido dojo, there's not a lot of intense weapon-to-weapon contact, and tsuba aren't needed. If however, the weapons work is more like that you might find in Shinto Muso Ryu jodo, for instance, then a good, stout (preferably thick leather) tsuba is essential.