Thanks Greg. Yes, I heard that as well. Whilst searching for an answer to my problem yesterday, I came across numerous references to it on R/C and Robotics forums, and nobody seems to know why! I can't help wondering if it's another case of "it's on the 'net so it must be true"!
Having said that, the consensus seems to be that it can't hurt so do it anyway, which I have done.
On the basis of one quick test, I think I have two servo's doing what I want, but I'm saying no more until I'm sure.