To MHO, I don't think it would matter, just make sure the plug is good (not cracked/chipped/burnt) but make sure the gap is correct and just use that. It won't make that much of a difference if you're using one that's platinum, copper or iridium.
The only difference is that coppers conduct well but have a short life span, platinums last 100,000 km but conduct less than coppers, and iridiums conduct well and last 100,000 if not more.
Don't worry, I mixed platinums with a copper one and I was good till I bought the platinum.
Its just better to keep them all the same.
Hope that helps
