I allow editors to enter content all the time. One trick is to add a container around the WYSIWYG, and to set any images to max-width:100%
tables are tricky, you normally have to break them away from it's normal table behaviour by converting the td's to display:block and possibly float them.
Typically, the editors will only enter text and add some images. You can't be perfect especially if the user is html savvy and can add some fancy stuff, but if you handle some problem areas that should be sufficient.
These problem areas are usually images, width's being set, and tables.