Well, I've tried walling off the north once I'm done with Humphrey, but nothing else. I'll try walling off the other places as well.
And yeah, I generally don't try lab rushes unless I'm completely out of options.
Although it is my usual strategy for the final level with Whitey. XDIf this current level is giving me so much trouble, I'm starting to get scared about the remaining levels. O.o
Julius especially frightens me...