I use a Retroflag MEGA Pi CASE which, when configured correctly, has a physical switch on it. I have a post outlining how to get it working on DreamPi:
viewtopic.php?f=3&t=11208&p=113551#p113551.
Before that, I used Termius to connect to it via SSH from my phone to execute the command, but that did still require me to unplug it and then plug it back in to boot it again.
Even before that, I left it on all the time. My back of the napkin estimate was that it would cost me $5 in electricity over the course of a year. I never had any problems with it working when I needed it.
Alternatively, you may want to try RaspController. I haven't given it a shot yet, but it does seem promising.
https://play.google.com/store/apps/deta ... controller