I use a similar fuse tap-based solution but with the additional step of running both the original (always on) wire and a new lead to an ignition-switched spare fuse socket through a 3-position (SPDT) switch. This lets me choose via switch position whether the outlet is always on, ignition-switched, or totally off. All 3 have turned out to be handy and for me the extra effort has been well worth it (those 19mm/.75inch round rocker switches are super easy to mount cleanly with nothing but a spade bit, and sit reasonably flush)