If the RF image isn't horizontally aligned at infinity, it means the rangefinder is out of whack. Infinity is the baseline point for RF alignment; if it's off there, it'll be off everywhere else, too.
Whether or not it will affect your on-film results will depend on how you shoot with it. If you mostly shoot at longish distances at moderate to small apertures, depth of field probably will camouflage the focusing errors and you won't notice any problems. If you shoot any close-up and/or large-aperture shots, you'll probably discover that the sharpest point is something other than what you intended!
If you really want to know before you invest any important shots, you can try the traditional 'yardstick test.' Mount a yardstick on a surface that you can set at a 45-degree angle to the camera; a door works well. Put the camera a short distance away from it, such as a bit over 1 meter. Set the lens at full aperture and choose a shutter speed that will give correct exposure.
Now, focus very carefullly on the yardstick's 18-inch mark (add a strip of tape or make a bold line with a marker to make this point easier to see in the finder.) Carefully make an exposure, using a cable release to avoid camera shake. Now advance the film, defocus the lens completely by turning it to infinity, and try again. Lather, rinse, and repeat until you've used up enough of the film that you don't mind taking it out.
Now rush out and get the film processed. If you cleverly used a chromogenic b&w film, you can get it done in a few minutes at any quick-service lab. (Same with color neg film, but I find b&w negatives easier to see.)
With processed film in hand, lay it out on a lighted surface and take a good look with a strong loupe. If the 18-inch mark is within the sharpest part of the picture, all is well. If your RF is off, the sharpest part of the picture will be ahead of or behind the 18-inch mark, and you'll also know the direction and size of the error.
This is a very demanding test, and it isn't unusual for a camera that seems to shoot well to be off slightly. Only you can decide how much error you can tolerate.
PS -- A Leica's rangefinder is of very high precision, so you'll be able to see tiny misalignments that might not be visible on a Canonet, even if the Canonet isn't actually aligned any better.
PPS -- Vertical misalignments at infinity are annoying, but don't normally affect focusing accuracy. It's the horizontal alignment you need to worry about.
PPPS -- If this is only a minor misalignment and everything else is fine... well, Leica knows (and all other RF manufacturers do, too) that RFs can drift out of calibration slightly, so just about every RF camera has a concealed means of adjustment that lets a technician touch up the alignment without major disassembly.
If your camera store does NOT know that, and if you know a technician who can adjust it, you may be able to bag a good buy on that M6 by showing the seller your yardstick-test pictures and saying "Look, the rangefinder is 'way out of whack! It obviously needs hundreds of dollars' worth of delicate repair work. But it impresses the chicks, so I'll buy it if you knock down the price enough."
