I think it's more common to address them using the names that matches their VATSIM callsigns (LA Center in your example) and include a brief hint as to which of their many functions is relevant for you.
For example, real world, it might be as succinct as "ground, N123, taxi." But saying "LA Center, N123, taxi" on VATSIM gives no context as to which of the numerous virtual tower cabs the controller should pretend he/she is in. Instead, "LA Center, N123, John Wayne, taxi" is a lot more beneficial.
My $.02.