How accurate are preview encodes with Vidcoder, Handbrake, etc?

[strawberryshortcake]

How accurate are preview encodes with Vidcoder, Handbrake, etc?

Instead of coding entire videos, are "preview" encodes a fairly accurate reflection of the final output file?

I can certainly see changes to the preview files when making setting adjustments (i.e. target file size, CQ value, bitrate adjustments, etc) with more pixelation and macroblocking when purposely using lower target output file size, or lower CQ value to find the happy medium on smaller file size vs. quality, and obviously better preview visuals when those settings are increased. But how "accurate" are these previews. If I am using a 2 pass encode, is the preview encode also implementing the same two pass technique to generate the preview encode?

Secondly: Is it possible somehow to save those preview files from vidcoder? I believe handbrake creates a real file in your folder, but vidcoder doesn't.