Meredith,
You can scribe some lines before you start assembly, it'll be easier to do so with parts like the horizontal stabilizers which could snap off under the pressure applied while scribing.
Also, lines that are near potential tight spots (which only manifest themselves after assembly) would best be scribed before assemly. These include areas where the horizontal stabs meet the vertical fin & rudder, wing roots and other similar spots.
I've only rescribed two models. But I've found that whether or not you scribe some lines before assembly, it's wise to leave those that cross any seams 'til last when you've glued, filled and sanded those seams.
Oh, and don't try to scribe through filler putty. More often than not it'll chip on you. When needed, I usually fill with superglue in areas where I know I'll be scribing. If you do use superglue as a filler, put it on and wipe it smooth fairly quickly with your finger, applying successive coats. Sand it within 10 minutes or so because when it sets up, it'll be rock hard and you'll wind up sanding more plastic than superglue.
As to any doubts you may have about wiping superglue with your finger, it'll dry on your skin and feel a bit funky, but if you do it fairly quickly you won't glue yourself to the model. I do this all the time. Heck when I get cut, sliced, ripped, torn or otherwise spring a leak, (anything but a huge gash) I close it up with CA.
If you do glue your fingers together or something, don't worry. Superglue has great pull strength, but very poor shear strength. So don't try to pull directly away, run the bond under warm water and work it back and forth and in circles.
I have my own method for engraving lines; if you care to check it out head over to
my meager website and go to the
How To... page. You may or may not care for my method, but I'm a great believer in getting second, third and fourth opinions...
Fade to Black...