For the price of a Ti frame, a little ti prep seems like a small bit of insurance to avoid a very serious problem. On the other hand, if you are a maintenance maniac and remove every part from your bike every year, then Ti prep is probably overkill. If you plan to leave the BB in place for several years, Ti prep will reduce the odds of it getting stuck.
Generally, Ti prep is critical when joining Ti parts together, like a Ti seatpost in a Ti frame. It's less critical when non-ti metals are joined to Ti, unless the parts will be together for a long time.