If you are looking at skimming the head, I would get it dimensioned first. If these are non interference engines, then something somewhere has gone wrong, possibly due to the following.
1. The head may have been skimmed before, possibly due to overheating or purely for tuning. Anyhow, material would have been lost bringing the valves closer to the pistons and thus losing the non interference properties.
2. You may have a high lift cam in there, again for increased performance. This may also cause the engine to lose its non interference properties.
3. A combination of the above.
Now if the head has been skimmed and you go and skim it again, you could take it out of limits.