F3G: can't disagree with that!
In my case during the 4 occaisons when I thought engine problems were going to end in a forced landing, I can say that I had my landing area and approach plan sorted before I did anything else and I believe reading a checklist would have been inappropriate. 5000',1000', 1800', and 400' AGL in case you're wondering and only one required a forced landing without power.
Therefore, I would take a punt and say use of a written checklist would be inadvisable below, say, 3000'(?) but committing to memory an engine failure drill to memory is a bl@@dy good idea at any height!!