I just want to complement Itai's answer.
A step-up ring is a better scenario (having a 77mm filter on a 72mm thread)
But when the difference requires a lot of rings, let's say you need to use 8 rings, the filter is now further away from the lens, and some flaring could be noticeable.
Of course, the pro is that for expensive filters you could buy just a ...
A step-down ring is required to physically mount the 72mm filter on a lens with a 77mm thread. Since this is a step-down adapter rather than a step-up, the field-of-view of the lens may become partially obstructed, causing severe vignetting or even appearing visible at corners of the frame.
Essentially, there is no pro for doing this, only downsides because ...
The six 35mm SLR mounts you can easily adapt to Canon EOS with simple ring adapters (no optical elements) that are readily available are:
The all have mounts deeper than Canon EF. (See also: Can I use lens brand X on interchangeable lens camera brand Y?). All film-era medium format lenses are probably ...