Connecting valve with shower and tub spout
I have been told that it is not advisable to use pex pipe between the valve and the tub spout, as it may result in pressure that sends the water to the shower. If the pipe is the same inside dimension as copper, then why would this be the case - unless the pex elbow being smaller might cause this ? I would like to plumb this without resorting to solder, and am trying to find out if it is possible. I also wonder if different brands/quality of valves might address this problem - I am using a new Moen posi-temp valve.
Also I have heard that the distance from the valve to the tub spout should be at least 11 inches and up to 18 inches. Does it matter ? Is it only about esthetics, or are there technical reasons for limits on this distance ?
Thanks.
Mike