Least Load doesn't seem to work for multi-configuration projects. I noticed this by creating a job with 10 configurations (5x2) while I had 8 nodes with 2 executors each in my system. All 8 nodes were part of a label called "Main" and the job was assigned to build on the "Main" label.
When I triggered the job, 2 of my nodes were completely empty while 4 other nodes were double-loaded. Given my setup, I should have only had 2 double-loaded nodes and no empty nodes.
I've attached the XML configuration for my job. If you create a "Main" label with 8 nodes, you should be able to reproduce this behavior. Heck, you should be able to reproduce it like so if you don't want to use my exact setup:
Nodes = N
Executors per node = E > 1
Number of configurations in project = C, where (N) < C < (E * N)
Given Jenkins' normal scheduling algorithm, it's possible you'll need to make C = (E * N) to fill your nodes, then add a node (or subtract configurations) to fit the above formula.