Do you fit Elements XPS panels tight to floor and ceiling?

[smallsteve]

When fitting waterproof XPS (Elements) boards in a shower recess, should they be cut tight to the floor and ceiling or should a gap left at one or both ends?

If a gap is to be left, which end should it go and what do you put in the gap?

TIA

 

[ShaunCorbs]

Tight as there no expansion required