[semi-stupid question] Reusing "Physical Wallets"

[johnniewalker]

Say you have a bitcoin bill, loaded with one bitcoin, with the address and key exposed. You redeem the private key on MtGox, and the balance of the bill is added to your account. Now, I know all a physical bill essentially is is a bitcoin address. Therefore, after you redeem the bitcoin loaded onto it, you could reload it by sending to the bill's address.

But is this smart? I typically redeem my physical coins on MtGox, although I know you can do it via blockchain and maybe even more ways. Once I redeem a private key on MtGox, and then I reload the physical bill, is it essentially the same thing as just having the coin in my MtGox wallet. Either way, MtGox has my key. So then it should be safe to reload physical...whatever, right?