As pointed out above, the problem is in the seals, ALWAYS!! Why do a valve job till you tell whether the seals will last.
I advise doing seals untill they don't last. They will not last once the guides are so wore that the valves move around to much or the seals can't stay attached to the guide. The good quality seals, MB uses, will seal worn guides for extended time. I would guess the guides would be worn by around 250-300k miles.
Bosch Master, ASE Master, L1
33 years MB technician