Although it's a good idea to have extra headroom for future upgrades, a smaller injector will generally perform better (than a large injector) at low duty cycle during idling and part throttle/low load situations. Injectors are easy to upgrade, so I usually recommend sizing injectors for what you need now, not what you think you'll need in the future. Big injectors can be a real headache, so it's better to save the headache for later.