As usual, it all depends –
If you’re talking about PC style mobos and switching power supplies, don’t worry about – just turn them on. If they’re bad, they’re bad. You may also find that they require a minimum load to even come up – either leave them connected to the motherboard or use 6 or 12v auto headlamps to load them down. Since switchers try to self-regulate, running them for too long below voltage will only stress them more than just turning them on. Wouldn’t hurt to use the Variac to sweep from 0 to 110v over a 5 or 10 second interval just to give you the ability to shut things off if something starts to smoke. You may also see AC filter capacitor failures (shorted) that throw out fireballs and clouds of noxious smoke. These are more common with Apple II and some DEC (BA123) power supplies. Nothing to be done – except maybe test outside.
On the other hand, if you’re working with vintage linear supplies, then reforming the caps may be worthwhile. Use an ESR meter to check first. Best thing is to remove the caps and reform externally but a slow bring up with the Variac (maybe in 20 v steps, a couple of hours at each step) won’t hurt. With these supplies, you may also need to worry about shorted transformers – here a 60 or 100W light bulb IN SERIES with the transformer may help you as a current limiter.
Lots of info on the web about reforming caps – check out the old radio guys. If you’re doing much of this stuff, an ESR meter will save you a lot of guess work.
You will also experience one or more tantalum cap explosions at some point in your adventures. Just keep your face at a respectful distance, at least during initial power up.
And remember to unplug any of your vintage gear when you walk away from it – those funky line filters are on the hot AC side.
Just empirical notes based on my experience – all the usual disclaimers apply.