NDAs are alive and well in Silicon Valley even if VCs don't get asked to sign them. I've interviewed with dozens (if not hundreds) of companies here and given product/engineering advice to dozens of other companies and in almost every case, once a real conversation is going to start they want a signed NDA.

I don't even think there's anything wrong with that, but I've not noticed any difference in terms of secrecy between the SV software companies I've talked to and software companies outside of SV (in places like New York, Seattle, LA, etc.)