If you are generating contours with CadTools you should use Interval = 1 and Minors per major = 1 as setting in the contour command, that would result in what you want. If you are annotating 3D polylines with the command "Annotate by fence" there are no settings for rounding.
It would be wrong to have rounding for contours, the annotation should be accurate if not its better to generate new contours with accurate elevation. As an option you could export contour annotations to Excel (Export Text to Excel). In Excel, round the Z-column and copy/paste X,Y,Z, Text and rotation it into "Draw from Coordinates" using Type: Text at point. Draw it into a new layer by input a name into the Layer list.
This will result in two layers one with the original annotation and one with the rounded values. Tip! You can change justification of the inserted text by selecting them all and then change the justify parameter in AutoCAD's properties dialog.
Hope this helps!
/Lars
