I did both. I dipped the ends about 150mm in a bucket of clear endgran preservative for at least 5 minutes and then (once dry) the ends had a couple of coats of Osmo. The thing I wanted to avoid was endgrain staining from water absorption so that's why I thought the Osmo coating was important to try and repel water, perhaps more important than the preservative?
As for pre treating, its essential to protect the timber and obviously you wont get to the back, the ends or inside the grooves once fitted, and if it does shrink a bit then you potentially have some unprotected timber. So I would not fit any until it was properly treated.
BTW, Wood Finishes Direct have an offer for the Osmo clear extra - 3l for the price of 2.5
Osmo UV Protection Oil Extra | Exterior Wood Protection (wood-finishes-direct.com)
As for pre treating, its essential to protect the timber and obviously you wont get to the back, the ends or inside the grooves once fitted, and if it does shrink a bit then you potentially have some unprotected timber. So I would not fit any until it was properly treated.
BTW, Wood Finishes Direct have an offer for the Osmo clear extra - 3l for the price of 2.5
Osmo UV Protection Oil Extra | Exterior Wood Protection (wood-finishes-direct.com)