I will tell you this if the plug is longer than the hole it does nothing for you in keeping it in the hole. However after 100,000 more miles it can strip out the threads. What happens is the extra threads on the plug collect carbon then get real hard. When you break the plug loose and back it out the carbon opens the threads in the head. Your best bet is to get them the correct length. Or better yet when you remove them turn them 1/2 turn out then spay some lube or cleaner down the hole and wait an hour then tighten them up LIGHTLY. Then turn out 1/2 turn and spary a little more wait 1/2 hour then back them out. You will not make the threads larger hopfully and you will avoid a blown plug.
This happens on aluminium heads all the time. Thats why they have helicoils.