In the Digital-Light bays, w/ LW 15/16, slots 1-4 were the "high-powered" slots because they were closer to the air vents / fans. I'm not sure that it makes that much difference in EL/ML architecture.
Still, if you have a slot available in 1-4, it's a good idea.
Mitel put out a tech bulletin recommending that you install it in the first 4 slots. I have found that getting in on the internal modem works better, less errors in getting data and programming.
Had one in slot 6 and got all kinds of random errors. Unable to access vm boxes, not a valid selection, not properly sending MWL. Moved it to slot 3 and problems went away.