**Usually** keeping filter caps charged is a good thing for their lifespan. If however that means that the caps also are running warm then its not going to help, especially if some of that warmth comes from ripple current (which tends to be higher at higher frequencies as seen in SMPSs). Careful circuit design can avoid these problems.
As one who has spent a lot of time studying the idea of going off grid and using solar power and the like, one thing that comes up in sorting out how much power you're going to need is how long something is on, and not so much how much power it draws. Class D amps are typically around 85% efficient so a 500 watt amp is going to draw some power even if its just sitting there at idle. Since there is never dead silence even when the preamp is off, that means the output section is engaged even if its not making a lot of power reproducing background noise.
I get that some solid state circuits sound better after they are on for about a day or so. IMO this really says that maybe they aren't all that green compared to tubes if you tend to leave them on all the time so they will sound right.