if your DM gives out +1 chain shirts and rings of protection a lot like most do... you may want to buy stuff that is less common and hope you can survive with 2 less AC. If you are fighting at range like a warlock normally does, then it shouldnt be an issue.
I found playing a warlock to get extremely boring at low levels in fights, because you are just blasting, so I started another warlock and went with stuff that didnt buff my EB at the low levels.
I loved the roleplaying stuff thou.
and...
Baleful utterence is very very powerful at low levels, as is the one that lets you summon a swarm! Certain swarms, like the spiders are immune to weapon attacks.
See the Unseen is very very useful if your DM likes to have invisible enemies... it really ruins their ambushes.
You can swap these out at higher levels for more useful invocations when they are no longer as effective.