putting aside the stress issues on the prop there is a reasonable safety argument here for not pulling/pushing the outer part of the prop. If you are holding the outer part you will inevitably be holding only one blade, if you then slip you will be far more likely to turn the prop and if you have a live mag you have an aircraft moving toward you or you will be falling towards the aircraft with possibly a very nasty outcome! By holding the prop at the root you will not only minimize stress on the bearings, blades etc you will also have far less risk of turning the prop.