As for entering in the flight plan take-off weight in the FMS as a cross check. My company would frown upon that.
It is also a matter of WHERE in the FMS you enter it...
In the Approach Reference page, the weight entry is temporary - it is erased as soon as you leave the page. Once the ZFW is entered into the Init Ref page, the Approach Ref page defaults to the current gross weight, but that can be temporarily changed to any value without affecting anything else in the FMS. The Vref values are directly calculated from that entry, though, and can then be entered into the FMS or simply referenced for purposes other than approach planning.
When we first got the 744, that method of crosscheck [of our paper performance charts] was also 'frowned upon', but as people started understanding the airplane (and we started moving away from paper performance charts 10 years later), it became an 'official' means of crosscheck.