Author of NGLess here (also MOCAT2, but that is no longer maintained, using it is not recommended).
Depending on which environment you are working on, NGLess may not require assembly. If there are already gene catalogs for the environment at hand, you can use them. Otherwise, yes, assembly is recommended.
Feel free to ask more detailed questions on the ngless mailing-list:
https://groups.google.com/d/forum/ngless
I generally would say that blasting each read is not a good practice nowadays: the reads are just too short. Even if you don't use our tools, I'd still ask you to not do it that way.