Saul,
if you consider the installation to be permanent, then the trestles do need to be rigidly attached to the track base. How far apart you space them is up to you, but anymore than six inches and you will have the track sagging under the weight of a locomotive. I really consider these as temporary supports only, when used on a temporary railroad. It will help rigidity if able to pin the trestle base to the railroad support base.
Thanks again Tim!