I just did an experiment: I put a 6x6 MF negative into my enlarger (a Philips PCS130) and projected 8x8-inch images onto my easel using a 75mm lens and several 50mm lenses (a Nikon f/2.8, a Nikon f/4, a Durst Neotaron, a Vega-11U f/2.8, and an Industar-96U f/3.5). The images from the 50mm lenses all exhibited some vignetting, although the extent varied from one lens to another. The worst were probably the Vega-11U and the Durst, while the two Nikons were probably the best. With the worst, the very edges of the prints appeared almost black (they'd print completely white).
I didn't bother to make any actual enlargements with these lenses, nor did I try measuring light falloff. (I do have an exposure meter, though, so I could do that -- but I don't think it's really worth the effort.) It's possible that an actual print would show substantial lightening toward the edges even with the Nikon lenses.
There's also the question of resolution. Even if a 50mm lens projects an image that doesn't fall off in brightness too much near the edges, it might project an image that's objectionably blurry in the edges. I didn't really look for that, though. Center resolution should be fine.
As I said in my earlier post, 75 or 80mm enlarger lenses are very cheap on eBay. Even a very cheap one (like Dead Link Removed or Dead Link Removed) is likely to do better than even an excellent 50mm lens for printing MF negatives. Such a lens will set you back less than the cost of a 100-sheet box of 8x10 paper.