I see a lot of research in very sophisticated side-channel attacks on crypto systems. Most (but definitely not all) seem to follow a trend, namely, the crypto system does something very dumb like decrypt any block given to it with no IV or nonce (so usually ECB mode).

That said, there have been practical attacks against smart cards and timing attacks against RSA.

So, when building a system which uses cryptography (be it public or symmetric key), how practical of a concern are such attacks? And, what guidelines can be followed to determine if your systems needs protections (be they power analysis protections, emissions, timing, etc)?