Be aware! I just picked up my new S9 from cross iron mills in Calgary and was told I didn't need glass protection film on this new phone because the glass wouldn't scratch. WRONG! I did get a rather deep scratch within a week. Telus won't provide any compensation for this incorrect information.
The S9 uses Corning's Gorilla Glass 5, one of the most scratch resistant faces for Smartphones. If you are one who is more likely to scratch the face of a phone due to the way you use it, you should buy a screen protector, no matter the recommendation of the salesperson. If you are like me, who keeps a pocket free of scratch-inducing content to hold my phone, then you may not need a screen protector. I had one on my first phone, but none of the dozen I've owned since, and have not felt the need to place one on my phone.
Ultimately, the sales rep can only offer advice, based on the information provided them. It is up to the customer to make the final decision.
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Beware of any and all claims that sound too good to be true. No phone is scratchproof just like no phone is waterproof either. Scratch, water, drop, and many other resistance ratings are all based on very controlled circumstances that don't reflect real-world use.