I can't say I've ever had seats sink in any FE I've ever ran, but I put hardened exhaust seats in my old 390 that I used in my '68 F-250 Highboy (25 years ago), only because it was going to be used to pull/haul heavy loads. I've seen sunken valves plenty of times in small blocks from the early to mid '70s; sometimes so bad I'd swear the exhaust wasn't flowing any air!
The heads I used for my Highboy were C4-G heads, and yes they lose quite a bit of support when cut for seats, but the guy who did the work was an old Ford guy, and he just made sure the interference fit was tight enough. Those heads are in use on my '65 Galaxie now, with nothing more than a light valve lapping before putting them on, so 25 years of use now with no issues.