No, Helix does not have consistent reports of mattress sagging. Of course, body impressions over time are normal. However, rotating the mattress every 6-12 months can help to reduce this kind of mind sagging.

Excessive sagging over 1″ is protected by the Helix warranty. The warranty is 10 years on the standard mattresses and 15 years on the Luxe mattresses.
To help reduce sagging, Helix recommends a solid foundation or a one with slats no wider than 5″ apart.