There's no fixed rule as to when and why, but in general if you don't have .500" of 90° drop from the SSR to the lowest seat cut removing any of it (relocating the seat deeper, even if the new seat cuts are identical) reduces flow - even if the SSR is untouched. If the SSR is affected, much worse.
Whether the local pressure, bulk flow, make this very important or relatively not I can't say but the default opinion on sinking valves is "unless you know it's safe - don't".
Based on the cutaways of the 250 and 292 heads I've seen, there's nowhere near the desired .500".
Unshrouding the head in the chamber is (in general) good, but it's not the same effect, and should be done in any case. The exhaust valve, on the other hand, is frequently unshrouded either too much or in the wrong place (also adds chamber volume).
