Binary Packages or Source Compilation for Gentoo Updates with Custom USE Flags?

Gentoo's Portage package manager supports both source compilation and binary packages (binpkgs). Source builds allow per-package USE flag and CFLAGS customization, while binpkgs reduce build time and CPU load. The trade-off becomes concrete when a system must preserve a small set of custom USE flags but also needs to shorten update windows.

The unresolved decision is how to treat packages whose local USE flags differ from those recorded in an available binary package. Portage's dependency resolver may prefer a binary package, compile from source, or report a conflict. The Gentoo Handbook describes binary package installation workflows, but the precise fallback behavior when USE flags diverge is not fully specified for all Portage configurations; verification would require comparing recorded binary package metadata against local USE settings.

For a current stable Gentoo system, which documented approach should govern: using binpkgs with default USE flags and accepting reduced customization, or compiling from source to retain flags at the cost of time?

• Under what conditions does Portage choose a source rebuild over a binary package when USE flags differ?
• Does the choice change with --usepkg versus --usepkgonly?