While it is true that heat is a prime killer of electronics, there are some other concerns, too.
Most electronic products are only tested briefly. Electronic components tend to either break down quickly or last for many thousands of hours or use. Since I don’t think anyone wants their synth to fall apart during a gig or even during a practice, it makes sense to find out if those components are going to last.
Therefore, what I suggest is running new products full time for at least a week. With synths, set it with a sequencer or other controller so that everything is working. You can simply not plug it into an amp so that it’s not bothering you or your neighbors in the middle of the night. If you don’t have any problems within a week or two of this type of activity, chances are the electronics will last a long time.
Most synths and electronics, today, are not purely electronic. They are also electromechanical. All of the keys, dials, disk drives, and switches are far more likely to have problems before the electronics. The best way to keep them working is to keep garbage out of them. That means no smoking in your studio. That means covering keys and switches with something, either a cloth, case, or old clean rag when you’re not using it.
Some parts are more likely to break down than others. Capacitors are notorious for failing. Keep an eye on the internet to see if a particular product has common problems and, if the product is out of warrantee, consider acting proactively.
If you have an older product, consider finding replacement parts. Top octave chips are becoming more and more rare. If you have an old synth which uses then, consider seeking replacement parts now so if it does fail you won’t have to pay outrageous sums. I don’t have one of those any more (I used to have a Korg CX-3), but I do have a Yamaha SY77 (which I consider to be one of the finest, most capable synths ever made). The SY77 and its big brother, the SY99, are famous for failed back lights on the LCD as well as for failed floppy drives. Get replacements now for those items. Do you have a Moog Source? Better find a replacement touch panel. Etc.
As to surges and lightning, consider where you live, too. I live in Southern California, and we haven’t had lightning where I live in years. If you’re likely to have lightning in your area, unplug. If you’re in an area where the electricity goes on and off and has problems, unplug.
I used to own a Farfisa “Combo Compact” organ. When you pressed on a key it would pull up a piece of plastic that would pull up contacts. But if you hit the keys fast and hard, or if the keyboard bounced around, the little pieces of plastic could become loose or break, resulting in a sound on continuously for the key. I eventually kept extra pieces of plastic and a long-nose pliers in my kit to make between-set repairs.
