When to recap?

Caps should be replaced. Why?...it's from the 1970s and even if it was treated with kid gloves, it's time to change them. And if it was in storage, the caps may have dried out.
You can look for yourself; see if any caps are leaking or dry and brittle.

Also, it should be bench tested by a pro and set-up to spec. I'll wager you'll need a few other parts replaced or tweaked; eg: resisters, solder joints, power suppy. After that, you'll have years of enjoyable use.

Hello Brad.

It depends on the caps. Electrolytics age - 40 years is very old for such a cap. I would definitely replace all electrolytics with new ones at this point, especially the power supply caps which come just after the rectifier. Many brands: I like EPCOS for the power supply - a German collaboration with local labour, I understand. Be sure and get new supply, because there was a problem with some manufacturers going cheap, or so I heard. Some amps can take more capacitance in the power supply, but others can't - depends on design. If you can't buy the expertise, replace with the same values (in microfarads). Make sure that temperature rating is at least as high. Digi-Key sells and has good service.

I have also read that Silmic by Elna are very good caps. Choose caps that have as low an ESR as possible, and don't stint the expense - you will hear it.

For caps in the signal path, you might want to upgrade them if they are electrolytic, improving to polypropylene or even styrene - but the problem will be space. Do NOT change the capacitance (in microfarads). Parts Connexion sells, and has good service.

I have no connection to either Digi-Key or PartsC.

Don't try to do it yourself unless you understand about stored energy discharge, as in caps and chokes. It's not hard to learn, just get a good tech to show you, but it can be lethal if you don't.

Former McIntosh repair tech, here.

Another clue; swollen tops of the electrolytics (can-type caps), and look for any dark liquid seepage from around the bases of the capacitors (some mfrs like NAD used a dark glue to help secure the cans, not McIntosh Labs).

All the internal PC boards WILL need solder joints retouched at their Molex connectors, and the carbon bias resistors (10, 12 or 15 ohms, depending on model, WILL have drifted if not replaced previously with metal film).

A properly biased McIntosh amp runs barely warmer than room temp even when driven moderately hard, so if the heatsinks are getting hot at all (or worse, one side heatsink warmer than the other), you need the bias circuitry resistors changed.

All the above will firm up the sound and re-focus the instruments better, as well as ensure future reliability and enjoyment.