when do you replace capacitors?

i replace them if they re crap caps. I like v-caps if you have room, the REL RT, or Theta caps. Big sound difference over stock caps if they used junkers.

Resistors are inert and don't really change over time (they can drift in value from thermal stress, but that's another matter).

Capacitors are a different story. Electrolytic capacitors are the ones to change; film types tend to last a very long time and rarely need to be changed. Electrolytics contain a semi-liquid dielectric compound that dries out over time (used or not) leading to catastrophic failure. Electrolytics tend to be larger in value (over a couple of microfarads); film type caps tend be much smaller in value (under 1 microfarad).

Variac is the porper way to restore a cap. If your electronics item is worth a lot of money, and you have the money to spend getting it variac'ed. Do it. Otherwise the turn on style will do "in a pinch" or if you are just too cheap to get it done right. It is a 'dirty' method, but works. (than means it is not approved by anyone who actually does this stuff for a living, because it cuts into the bottom line)

Not because it cuts into the bottom line, because it's much safer.

So, the equipment is going to "explode in boiling fire"? Or is it that the cost of replacing a blown cap is less than renting a variac?
I can understand if the item is a bit of medical equipment attached to your heart pump. but usually old stuff that was stored for ten years is not the usual Ferrarri material.
I would not be afraid to use the cheap and dirty method on ANY product i owned.
Any yeah, if the product means a LOT to you, and it is really valuable, then splurge if it makes you feel better. But the item is not going to explode with the turn on method any more than with the variac.
This is like do you take your car to the dealer? or a local guy who you know.

Perhaps I just like to live dangerously?
I deliberately bought a SinglePower MPX3 because the notorious 'exploding with brimestone and boiling fire, killer/blinding headphone amp was just too good a conversation piece to pass on. Aparently some dude over at headfi got in a tizzy over his product being shoddily made and declaired all SinglePower products to be timebombs waiting to maim and kill the owners, and everyone needed to stop using them immediately, or risk immolation.
I have used my SinglePower now with it on the higher bias setting and so far have not been killed. (though some wish I was dead..sigh.)
Or is it I just am practical?
I stand up for cheap HiFi. and the turn on is fine by me: cheap.