I have repaired hundreds of MB heads (or my shop has). We use the factory go-no go gauge for guides to test randomly the guides we are replacing. Heads with 200k regularly pass.
We have put valve guide oil seals on at least a thousand MBs. We have never had a case where our risk taking was proven unjustified. I submit that by taking no risk, requiring complete resuurection, for each case one can never know what will work. I have striven to find these lines and have taken the risk for my customers to judge when they should drop $800 or $2500. In the case of valve guide oils seals I have never lost, point blank.
I aproach driveability diagnostics the same way. I strive for the single cause. I loose quite regularly. There is probably nothing quite as cut and dryed as this VG oil seal issue. I'm sure I could make more money by being 100% sure and doing the $2500 job every time. I could even do better if I charged $4800 for it. I'll have to work on that part. But, I do this cause I like it, and hitting the target without a shot gun is what makes it fun.
If the medical industry did things my way medicine would cost less than 1% what it currently does. Unfortunately we would bury the losses. I've done real good at this for a long time. I wouldn't recommend it if one wishes to make money.
Bosch Master, ASE Master, L1
33 years MB technician