Dwight and Miguel,
thank you very much for taking care of my case.
My drawings are already made up by 3d elements only (no lines, just some fills here and there to show particular things like site perviousness and such). I'm working on 3d BIM models, if that's what you're telling me to do. That was my main goal for switching from Autocad to Archicad (and from the 20th to the 21st century, if you ask me).
It just happens sometimes that some error may occur in creating/editing those 3d elements (walls, slabs etc.), so that alignments go lost, angles are not totally precise... I guess it's more a snapping issue than an "ortho system", really.
One more thing I miss from working in Autocad are the snaps. I guess I still have to learn how to use Archicad ones at best.