Hope I can help here,and I hope I don't tell you wrong,either,but anyway,...on an amp with a standby switch if you're going to play for about fifteen to twenty minutes a stretch and stop to take a break,I would just put the amp on "standby". On an amp without a "standby" switch(Fender Blues Jr,for example,...I know,I had one before),I would just leave it on and turn the volume way down,...at least that's how I done it. I do this with my amateur radio equipment,even though it's a solid state modern radio,...if I'm going to take a short break,I just leave the rig on and turn the volume down,and when I am ready to start up again,turn up the volume and continue. But,...if you're going to be away from it for a much longer period of time,say two or more hours,I would go ahead and power down! It all depends on how long of a break you take between practice sessions,..if they're short breaks,leave it on and put it on "standby"if equipped with standby switch,...if no standby switch,leave it on and turn your volume down. I could be wrong here,but I would think that powering up and down a lot does more harm than just leaving it on. As with a light bulb,..constantly heating and cooling the filaments in a tube by powering up and down a lot shortens the life of the tubes. When you know for sure that you are completely done for the day,by all means,power down all the way. What make and model are your amps,by the way?