Block_0.png is no longer used for machines (since IC2 released, if I'm correct). The machine tiles there are basically a legacy, leftover from the old way but unused. The sub-sheets such as block_machine, block_generator, etc are used for those items.
1st Row: Bottom
2nd row: Top
3rd Row: (probably) Back
4th Row: Front
5th Row: (probably) Left
6th Row: (Probably) Right
I'm sure on the top, bottom and front... but the other three sides tend to be alike on the machines so if you wanted to customize those you might need to resort to a little trial-and-error.
Also, the first 6 rows are when the machine is in the "off" or unpowered state.
The second set of six rows are when the machine is "on" - receiving power and activated to perform a task.
Machine Block
Iron Furnace
Electro Furnace
Macerator
Extractor
Compressor
Canning Machine
Miner
Pump
(unknown)
Electrolyzer
Recycler
Advanced Machine
Induction Furnace
Mass Fabricator
Terraformer
I hope this is helpful to you.
You know, I think you're right! I was trying to think of a machine I'd forgotten - now I understand why! 
