Keep a very close eye on it - as soon as you see a hint of green (germination) whisk it off and leave it off. This is usually done to speed up germination and to keep the birds off till it starts growing.

Never heard of that before. Unusual.

Yes me too, have sewn many new lawns over the years, never had to use polythene.

I've used it a couple of times years ago when there were young children at the garden - early in the year it does speed up germination and serves as a timely reminder to not run across it.

We usepolyester fleece occasionally for the same purpose, here, but we check daily for sprouts, and (as Bamboo says) whisk it off as soon as we see some!