If you're powering it from a battery, you might not want to do that. In any case, you pay for kWh so you may, or may not, think consuming a couple of watts 24/7 for 365 days of the year is a trivial sum.
Personally, I wouldn't use a Rpi unless it could reliably go through a normal boot shutdown cycle once a day. Use the Volumio interface to turn if off which actually just halts the system. Then wait 30 secs to be safe before switching off power.

