Adding some long-term closure to my own ancient thread, in case anyone else ever comes across this in a search...
I started building my FT5 a year and a quarter ago, but after almost finishing the mechanicals, life got busy and I ended up putting it on the shelf until completing the electrical work and powering it up just this week.
I did end up installing a handful of the aluminum corner brackets when building my FT-5. Based on the discussion above, I didn't see it necessary to put any behind the melamine, but instead I put four around the inner perimeter of the top of the frame to help keep the top square, and I put four around the inner perimeter of the second level so that I could remove/adjust the long rectangular melamine parts that hold the top of the Z rods without having to also remove and then re-square the Y-axis rails. For better alignment, I used the larger aluminum corner brackets, the type that take four bolts instead of two.
So what's the verdict? After over a year without being used, ALL of the bolts through the melamine were loose and needed to be re-tightened. I don't know if it is because the wood crushed a bit, or because of how many temperature and humidity swings this machine has been through (90+ summer and below freezing winter), but none of them were tight anymore.
The bolts on the aluminum brackets, however, were just as tight as the day I installed them. Not a single one needed any attention.
I realize that this is a somewhat unusual case - long term no-use storage in a non-conditioned space. But it's clear to me that those aluminum angles have completely different physical characteristics than the melamine parts, and I'm REALLY happy I put those aluminum braces on when I did.