Dave,
I find things such as wheel bearings last longer if you keep the bearing seals intact for the very reason you stated, less likely to contaminate the factory applied grease. I replaced my Mark 3 front wheel bearings with modern ball bearings with double seals some years ago. I left the grease fitting in-situ, but were anyone to grease it, it would only harmlessly fill the space in the hub between the bearings.
If you need a better example, how many modern motorcycle wheels are out there today running on permanently sealed ball bearings with no provision for greasing?
Do smear a film of grease on the internal parts when reassembling with sealed bearings, so they do not rust!
I am not familiar with the "U1" notation as a tolerance, here in the states tolerance or precision is usually denoted by ABEC numbers, ABEC 3 being general tolerance (or looseness!) ABEC 5 being a cut above, and ABEC seven used for precision things such as grinding and machine tool spindles. Even the crankshaft bearing on the Dougie does not need to be any special precision bearing above and beyond the norm. But it might well pay to select one that is a 'maximum capacity' designation. And the "U1" designation might refer to that. Maximum capacity bearings have extra rolling elements (balls, rollers, ect.) for increased load and/or longer fatigue life. Often they require slots cut into the inner and/or outer race to load the balls before the cage is installed. Do you have the maker's name for the bearings (other than the country of origin), as though the 'core number' for the size are industry standards, the designation for the options such as shields, seals, none, tolerance, extra capacity tends to vary manufacturer to manufacture (the subsidiary numbers you noted.)
-Doug
