If you can taste it and/or smell it, you should declare it as a specialty ingredient.
Since lemongrass is somewhat delicate, however, you should taste a bottle of your beer before you send it to competition. (Always a good idea, anyhow.) If the aroma and flavor has dropped, or it's masked by another ingredient, then don't declare it.
Likewise, if an ingredient only manifests at low levels in flavor OR aroma, but not both, think twice about declaring it. Judges tend to take off points if an ingredient only comes out in flavor but not aroma, or vice-versa. In particular, if they get a nose full of an ingredient and it's not there in the flavor, they'll probably ding you for it.