Shamus,
In the 'old days' when I hand-layed track, I used them where American prototype railroads did--across from frogs, on bridges and viaducts, and in situations where great harm could come if there were a derailment.
Not being an expert, I wasn't sure what the rules were, so I just did it where it would look good. Naturally, the one big accident I had (HO articulated loco taking fatal 5 foot plunge to a concrete floor) happened at a point where I had completed neither scenery nor guard rails. After that my focus became to at least complete a secure net of hard-shell scenery base before running trains. Haven't had a major calamity since.
I don't know if the guard rails help or not, but well-installed ones do look good and don't seem to hurt. Now I use commercial turnouts and flex track, and I DO install guard rails on the bridges I build.
What's your practice?
Bob
[This message has been edited by BobMcD (edited 01-12-2001).]
[This message has been edited by BobMcD (edited 01-12-2001).]