Very hard to say, sgfxi adds in several nomodeset methods, and if you used only one of them manually, you may find you don't have enough.
Also, sgfxi does not ONLY add one nomodeset, it also rebuilds initramfs, and updates xorg.conf, then says you need to reboot. If you manually add only a part of this process, sgfxi will assume the entire process is done, which would mean that it would not do anything, which would mean the incomplete manual changes would remain incomplete, which might mean a blank gui or blinking cursor on reboot. An LMDE developer will have to step in here and volunteer some debugging time I think to help resolve this issue, there's no easy way other than test, retest, and because this is a video driver, we can't use a virtual machine to do the testing.
Sometimes you should not reboot but halt, shutdown, wait, then restart, which is why sgfxi tells you to do this, this is because sometimes things are left life in ram, like initramfs, which can contain the nouveau driver, and a reboot doesn't clear the ram. Hard to say.
I'd have to receive full /var/log/sgfxi/sgfxi.log files for each step of the way to be able to see what's actually happening here, until that happens there's not a huge point in wondering.
Some cards may have issues, but I promise you, nouveau is a pain in the a$$ to deal with, and anytime I step into serious debugging work I have the pleasure of losing at least one day, often two, in the process, unpaid, unfun, and just generally a waste of one's finite life time.
I hear rumors that xorg 1.9 nouveau is actually usable now, by the way, but the way the kernel guys designed this stuff to lock in to the core system is not cute at all, and violates everything I have seen that is good about the linux kernel, in fact, it resembles exactly how windows kernels deal with drivers, you know, reboot.... reboot... pray... reboot....