Using all capital letters is usually reserved for acronyms such as SPICE (Simulation Program with Integrated Circuit Emphasis). Spectre is not an acronym, as such, only the first letter should be capitalized.

By the way, the name stems back to the days when Spectre was a harmonic balance simulator being developed at Berkeley. Originally it was named "Harmonica", a play on harmonic balance. Unfortunately, Compact Software decided to call their program "Microwave Harmonica". To avoid confusion, I decided to change the name. At the time, I have been talking about the program at conferences for some time, but had not yet released it and had given no date when it would be available. So I decided to call it Spectre because it meant "spectral apparition". The term "spectral" is a reference to the frequency domain nature of the harmonic balance algorithm. The term "apparition" basically means phantom, or "something apparently seen, heard, or sensed, but having no physical reality".  It basically pokes fun at the fact that the program was not available.

Cadence eventually acquired the rights to the Spectre name, and the name of the Berkeley version was changed to hbsim.

-Ken