I've always allowed it. I think massage is one of the best forms of foreplay because you get to explore the other persons whole body. It is also a great 'ice breaker.' The client might not be sure if mutual touch is allowed so if I act upon it first its could really easy up any tension or worry they might have about mutual touching.
There are exceptions like if im working on a certain muscle (like the arms, shoulders or back) but they want to move just to grab me it could be cumbersome because the body isnt in the correct position anymore! Plus I wont be able to work that specific muscle fully.
Plus if theyre in middle of touching me and i have to move i just keep going even if it means that they are out of my reach.