In reply to andy kirkpatrick:
Ah, it depends.
I know it is not an issue in this country, but in New Zealand you occasionally have to bush-bash to access cilmbs. When pushing through thick bush you need as streamlined a pack as possible, so I put axes on the back of the pack. I have ripped a few axe attatchement points off doing this, so they need to be strong.
For attatchments to the back of the pack, I find bungy a bit bouncy and prefer something secure. Macpac's webbing and buckle solution works well. Or a clip, or bit of string and a knot. It doesn't really matter. I have not used velcro, but don't see the point. All this talk of being able to access an axe easily seems pointless to me. If i need it I will take my pack off and get it.
On a long walk in with a big pack I will put the axes down the side compression straps as it keeps the weight balance closer to your back.
If I am later using the same big pack on a climb, and it is empty, the floppyness of the pack means the side method is less secure and I will revert to the back of the pack method.
As an aside, the first 'alpine' trip I was ever on, I slipped on a log and impaled my forearm with the pick of the ice axe on the back of my pack. From now on I ensure the pick faces inward and not outward. It does matter.