I usually aproach the job with seals up to about 150k. We always remove all plugs and do the cylinders first that show the most oil carbon on the plugs. We do the seals untill we find a guide that is beyond tolerance. That tolerance is based on experience as it can't truely be measured with the valves in the guides.
We look for tightness, condition of the seal retaining groove and the movement of the valve in the guide. The later seals will accomodate lots of play; burning a valve never happens. The whole job pays 5.5 hours. With seals and gaskets over $400.
Bosch Master, ASE Master, L1
33 years MB technician