You can also override PKGEXT as an environment variable when invoking makepkg, so you can have compression by default but easily skip it when it matters:

  PKGEXT=.pkg.tar makepkg

EDIT: By the way, it's often a big win to use multithreaded compression (pigz, zstd -T0, etc.) in makepkg.conf. With this, it's fast enough that I hardly ever override PKGEXT anymore.