You heard correct, shaving the head will cause the compression ratio to increase which leads to high temperature burn, In OHC engines the problem of mistimed camshaft also occurs due to excessive shaving/facing - meaning the total height of the engine reduces which then causes the belt/chain to sit a bit slack - when tightened by the tensioner the timing moves because the reduction in overall height is quite low compared to the tooth gap of the belt or link gap of the chain.
I think the problem is two fold.
1 - mechanics are not qualified now - as you are in this business you also know that if you start to take out head bolts without proper sequence you will crack or warp the head if you dont loosen them in the right pattern and in the right steps (at least 3 steps in TTY bolts)
2 - the machinists are there to loot money - they will shave any and everything to make money. The mechanic not being qualified cannot help and the machinist is in agreement with them to get business which also involves mandatory machining to run up the bill.