That's your call :)
Keep in mind you're not required to move up at all. If Titan 0.5.4 is working out for you and you're not running into showstopping issues, you can certainly stick with it. I think I've heard of projects that are in still in production on Titan 0.3 and 0.4.
Upgrading from Titan 0.5 to Titan 1.0 isn't necessarily a simple task because there were major changes between TinkerPop 2.5 and TinkerPop 3.0.1. But once you've moved up to TinkerPop 3, the excellence of the TinkerPop graph platform kicks in -- you have many migration options. See list of Graph Systems on the
Apache TinkerPop home page.
-- Jason