This subject probably would garner more varied responses in the Painting Forum, but anyway...
Gotta "Ditto" Hawk... Too many times I've seen them WAY overdone, and have even been guilty of it myself... If you're building your model to be looked at with the Mk I Eyeball, it's best (IMHO) to give them a wash in a contrasting color and leave it at that, rather than shade them so heavily that they look like they're a scale half or three quarter-inch GAP rather than a butt-jointed seam...
Some panels need it, here & there, like the joints of engine cowls on say, a P-51 or a Corsair's cowl-flaps, as well as hinged areas like flight control surfaces and the like, but generally, I limit it to post-shading with pastels (on raised lines, that is) to impart an indicator that a line is there, however, I'm just not very good at it yet, and have a tenedency to be a bit heavy-handed...
For models with recessed lines though, I really don't see a need to emphasize them with pre-shading... they are as Hawk stated, a waste of time (and paint) for beginners and even experienced modelers like me, although I've seen some of the "Master" Modelers pull it off quite well, with subtle but distinct shading in just the right places... I've yet to master that technique, since I seldom listen to the voice that said "STOP!" on the last pass, lol...
I guess that's where we get into the grey area that blends "Artist" and "Modeler" together... Of course, if your technique is done with photographing your model as the primary reason for your shading and coloring techniques, then we're into another area entirely, that being the technique of taking a 3-D object and making into a 2-D object, but painted and shaded in such a way as to turn it back into a 3-D object...
BTW, I'm a firm believer in "Post-shading" with pastels rather than paint... Used as a weathering technique, it's way more controllable than pre-shading, and if you're relatively new to the hobby, or even if you're just wanting to try something different, you can fix it if you screw the pooch...
Man, I over-thought that one... Percs are kickin' in guess...