That all depends on the whole picture. The Amp, Guitar, and the tones you're going for. So, there's no hard and fast rule here.
Generally, I prefer the power tubes to be working hard. But, you can't do that with every amp, in every situation.
Of course, having different amps, with different power ratings, is ideal. i.e. Maybe a 3 to 5 watt "Champ" styled amp for the living room/bedroom, a 15 to 20 watter for band practices and smaller gigs, and maybe a 30 to 50 watter for larger situations. But, not everyone has that luxury.
A couple of other options:
I find that a Timmy pedal allows me to retain some of that "on the edge/slightly breaking up crunchy rhythm tone" that you lose when turning an amp down. Very cool pedal for that use.
Also, an Attenuator. This lets you crank up the amp, yet "soak up" a lot of the power before it hits the speaker. This keeps the volume down, while the amp gets to run free.
So, in my opinion, keeping the Amp's volume higher is preferred. But, there are other ways that people have come up with.
Does this help?
Early 80's 1957 U.S. Vintage Reissue Stratocaster (Surf Green)-Warmouth Soloist Pearly Gates Neck, Pearly Gates Bridge- Larrivee D-03 (Mahogany/ Spruce)
Carmen Ghia Head- Marshall 112 Cab W/G12H 30 or Custom 4 X 6v6 Head or Budda Twinmaster Plus Head-Traynor 212 Cab w/Eminence Texas Heats.
Mo'D-Eternity-Blues Pro- Timmy-BYOC Chorus/Vibrato- TC SCF- Korg DT 10 Pedal Tuner