Hugh, i have no knowledge about that, but as i knew luk is not weaken the blade as long as the Empu (person who make kris) have good work & quality control on his kris..
(remember, in ancient period an Empu can only produced 1-2 special Kris in a year!)
Mervyn, i haven't study much about Kris's history, but in fact most of the Kris style (the 'dhapur' like the pictures above) were designed during Singosari Kingdom (12-13th century) and Majapahit Kingdom (14-15th century)..
Majapahit was a great kingdom and occupied most of Indonesian Archipelago, Malayan peninsula, and Southern Philipines..
All of that area were influenced by Majapahit style of Kris, but after the Majapahit kingdom collapse, the little kingdoms which used to be occupied by Majapahit developed their own style but still include the basic Majapahit style..
Singosari and Majapahit kingdom were a Hindu and Buddhist kingdom, not a Islam sultanate..
Java, Moro, Bali and some other island also have a huge Kris (with blade more than 70cm!) but that's very rare..
Burma, Thailand, and other Indochina area were not occupied by Majapahit, so they were not influenced by Majapahit style.. :)