You can visit Sewer Access after passing by Lab 16, but you won't actually need to do so until much later in the game. On the other hand, there are some very nice Treasures to be found here, and the Monsters are suited to the party's level. Later on, the Treasures and enemies will be trivial. It's a worthwhile sidequest, and the return trip is quite easy since your main goal is to open a shortcut through the Area.
Heed the note you find on the lower level. Don't touch anything, including the save point, or you'll be attacked.
The Boss, Krawlie, has a nasty attack that reduces someone's HP to 1. Heal them right away and keep up your offense.