Right on... using an O² algorithm when a well-known log O algorithm is available on serious amounts of data can easily make CPU cycles matter. However, that could be construed as just knowing what you're doing.

As Donald Knuth said (paraphrasing Hoare),

"We should forget about small efficiencies, say about 97% of the time: premature optimization is the root of all evil."

So, dragonchild is mostly correct.