I have a HK-TC30, but I would think that all the rechargable remotes are similar in this respect.

Is it best to return the remote to the charging cradle ONLY when the battery charge is low, or can / should the remote be stored in the charging cradle with constant charging ?

If the HK-TC30 uses the same rechargeable batteries like the standard Harmony series, it's a Lithium Ion battery. The battery doesn't have a memory like Ni-Cad, so you can recharge it however often you want.

I charge my 880 whenever it gets to 1 bar because I'm lazy to put it on my charger. My friend charges his 880 every day. We both had them for around 2 years. By the time the battery is truly dead on either, I doubt the buttons will even work on the remote.