I have read this in two manuals now, CLS' A340 and 742, it states that the pilot should not allow the plane to stop when turning during taxi. Is this true in the real world and if so, why? Just curious.

Depends on the aircraft but in most cases yes it is.

When I was in the USAF I worked on C-5s, C-141s, C-17s, and also helped to guid KC-10s, KC-135s and C-130s, 747s and DC-8s ect into parking...

A Cessna 172 would probably be okay to stop in a turn,

However a larger aircraft such as a C-5 or 747 it would be a bad idea. Espessly the C-5.

This is because it puts enourmous stresses on the landing gear. If you look at the boogie, or the gear structure, you see its got wheels in front of wheels attached all together, They dont turn or pivit. C-5s and 747s doe have a way of letting the gear as a whole turn, but not the indivual wheels on each one. This twisting force can roll a tire off of a rim if your not careful. We were always told it had to either taxi or be towed straight for 50' after it comes out of the turn.