Technically, Nickel runs @ 32bpp, so it's happily blitting to the framebuffer in BGRA, and leaving the EPDC/PxP to do the grayscaling, followed by a decimating quantization pass (unless you're on a Mk. 7, in which case it does an ordered dithering pass).
Which means feeding a grayscale image to Qt nets you an implicit conversion to RGB32 behind the scenes 
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Note that, as was mentioned earlier, there are probably a number of circumstances where you won't be able to avoid scaling entirely (even if it's for something stupid like a 12 pixels margin or something). Still, you can't go wrong with the native screen's resolution, because at least you can be sure that nothing will ever ask for *larger* than that
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That said, we've pretty much all agreed on the fact that both performance and experience in an image-heavy CB(Z|R)/PDF/(K)ePub was going to be fantastically awful in Nickel anyway, so, meh.