new build opinions required

[Jonathan]

I'd actually go has far to say it's potentially less accurate because your soley relying on the epoxy with no room for error or improvement where has with shimming you can tweak out any error it just takes a lot of time.

The problem with shimming is you can't compensate for rail twist with it. I discussed it at length in the recent build log. I didn't include the readings from the shimmed rail in that post, however if you want I can dig out the graphs obtained when the rail was shimmed and you'll see the error was still far worse than the final result obtained using self-levelling epoxy. I'll quote myself, from the build log:

At this stage it is interesting to note that the bearings slide smoothly on the rails, so although compensating for the height error with shims or a machined profile is sufficient for the bearings to run smoothly, this does not by any means assure accuracy since neither method compensates for the angular error.

It all depends on how accurate you want the machine to be and its intended use. The error at the tool introduced due to rail twist will be exceptionally small, so you'd need a very strong machine to notice it. The additional wear on the bearings caused by the rail twist will depend on the stiffness and accuracy of the structure they're mounted to, so it's hard to judge, but in a DIY CNC situation is unlikely to be too significant. I'd still choose epoxy though as it's so much quicker and easier than messing about with shims and enables one to get very good accuracy with less measuring equipment.