Resonant filters can be very loud, silence is a safe default.

Yes, yes, but at a Q of 1 (it's default) it isn't especially unruly or loud. ResonZ is a bi-quad, not a 303 set to "accent" ;-), especially at it's default Q of "1".

Moreover, defaulting to any common (or uncommon) note could hide a missing initialization, possibly much later ("Hey, my complicated filter should be tuneable and it isn't! It's a bug! Help me!").

I suppose so, on some level, but if you're building a complicated filter yourself you likely aren't accidentally adding in a ResonZ set to zero Hz either, nor does filters set to 0Hz make tuning anything much easier. If you're doing that you'll likely using adding BiQuad anyway.

Third, a filter is expected to be set to an user-specified frequency and no default is reliably appropriate; so why bother second-guessing what the filter inputs would be?

With all due respect to your thoughts above, this point seems the core of the discussion to me. My answer, and the reason why I brought it up at all, is that oscilators *are* instantiated at arbitary frequencies. This is in fact quite usefull if you are livecoding or for some other reason setting something up realy quickly and want to get a sound imediately to be fine-tuned later.

My point is that many things if left (temporarily) untouched default to reasonable asumptions, for example everything defaulting to unity gain. Unity gain can be speaker-damaging loud but it's a sensible default. There's some merrit to your line of thought, we could have oscilators default to 0Hz at a gain of 0 and make people turn it up themselves. That's safe in a way but very inconvenient in practice.

Yours,
Kas.