That's true, focusing at the sensor plane doesn't require as precise a tolerance of the registry (as opposed to a DSLR with optical viewfinder and focusing screen) however, this doesn't necessarily imply that there is such a variation.
YES
Bottom line, adapters should be engineered to focus as slightly beyond infinity, but as slightly as can be. This way close focus isn't compromised.
All the adapters for FourThirds SLRs I've used also allow adapted manual lenses to focus slightly beyond infinity too. Including the Olympus OM to Olympus FourThirds adapter.
The issue is this: Film camera lenses and tolerances in mount registration were designed for film's characteristics. Film is not rigid, it is variable in thickness, and to make it "mostly flat" yet still allow it to pass through the camera without scuffing, peeling or dragging required quite a bit of tricky engineering, with many solutions over the year. The most common, modern solutions allow the film in a 35mm camera to curl a few tenths of a mm in the format gate so that the center is approximately at the registration distance, the acceptable variation being up to a couple of tenths of a mm plus or minus (generally plus to accommodate the thickest films). Film camera lenses were designed to accommodate this variation in film thickness and consequent registration distance. (See Norman Goldberg,
"Camera Technology, The Dark Side of the Lens" for a larger discussion of this subject.)
Digital sensors are both rigid and far flatter than film ever could be. So the registration distance values in digital cameras are actually much tighter than they ever were for film cameras. Unfortunately, film camera lenses don't know this and many image best at a nominal registration +0.1mm or so distance, and that's how their infinity registration is calibrated.
As a result, when placed on a digital camera mount machined to the precise nominal registration, these lenses will not actually achieve infinity focus. The only way around that is to make the mount adapters a little short of nominal registration distance to ensure that adapted lenses will achieve infinity focus. Most achieve a little more than infinity.
There's no lack of precision in digital cameras with respect to mount register. The variability in mount register is due to lenses designed to accommodate the necessary variability in film camera mount register.