On Wed, 19 Aug 2009 15:29:18 -0700, Salmon Egg
wrote:

Does it make practical sense to preemptively replace anodes in working
water heaters in order to extend their lives? Is it better to just wait
until leaking begins?

Bill

Changing it after it starts leaking is not going to help.

Changing it before it is all used up can certainly extend the life.
The trick is knowing when. The second trick is actually changing it.
Can be a real job to break it loose without damaging anything, and you
need a lot of clearance above to replace, although you can find chain
type and/or segmented ones that work for low clearance.

Some manufacturers are using a combination inlet/dip tube/anode that
require disconnecting the supply piping to remove.

HTH,

Paul F.