Rhino commands (and grasshopper components) and tools work to a tolerance that is specified by the user within each Rhino document, and it's quite important that designers are aware of this and have given thought to the tolerance before they start modelling. Because Rhino is used in so many industries and purposes, the installed defaults should be edited for your purpose. You can save your own defaults by opening a new document in your required units (i.e. metre, or millimetre etc).
The default tolerance is specified in your Rhino Template files but can be edited through Tools > Options menu.
Problems due to a "loose" tolerance can emerge at later stage of project design, particularly when the model is exported and used as an input to other uses such as rapid prototyping, or Finite Element Analysis. It is recommend setting your tolerance as tight as you dare, and relaxing it when necessary if commands such as the boolean operations, intersection, splitting etc are failing.
Please also keep in mind that when exporting files (for example, export to IFC) with the Geometry Gym plug-ins, the active rhino document nominated tolerance will be used to determine the precision of lengths when writing out. If this is less accurate than the import software or project requirements than this can be made more precise by changing the Rhino Tolerance (or precision).
For further information on tolerance here is the Rhino wiki entry on tolerance.
It's common practise in CAD to model construction projects in worldwide position (ie OS coordinate system).
In Rhino, it is strongly recommend to keep your model in space close to the origin by using a local project axes system. You will likely observe display problems if you use a coordinate system with coordinates of a large value such as those typical in OS. This is because Rhino uses numbers of "single" precision for display mesh positions.
You can set up named construction planes (similar to User Coordinate Systems in AutoCAD) that will enable you to interrogate or specify coordinates in your model in large coordinate systems.