The IW trailers have crissy-crossy floors which provide very good grip without anything added, but as well as being more comfortable if they decide to lie down, it'll be easier to clean out if you put down some straw or, if you have it, sawdust / shavings. Not too much, just enough to help them grip and to absorb some of the wet.
We always say it - it's only stupid to
not ask!
And another tip - sweep it out and wash it down as soon as you get the sheep out - it dries very quickly and sticks like glue within a very short space of time!