> Is a bivy necessary at all times under a tarp?
I'm definitely not a tarp camping expert, but I would say no, definitely not at all times.
In the Cascades in summer, bug-proofing is usually a major issue unless you get some stormy weather or a cold snap. The mosquitoes around here can be quite terrible, especially on the western slopes when the tarns are full and the days warm.
My solo bug shelter is an MLD Serenity, my two-person bug shelter is an HMG Echo II insert. If you camp with a tarp, it's pretty easy to string up a bug shelter under it, and with modern fabrics the bug shelters are pretty light. I prefer to have space to move around and if necessary reload my film holders within my shelter, so I like the 2-person cat-cut tarps, since even when buttoned down for storms, they're still pretty spacious. It's too bad the Cuben ones are still so expensive, though.
I've been out on a few nights when I would have skipped the insert had it not been for the bugs, and a couple of nights where the insert's walls, even though they're low, cut the breeze enough on a chilly night to enable a comfortable night's rest... which was continually interrupted by a LOUD snorer :-/