Should I leave spare U for future expansion, and how much?

Yes, where space and budget allow.

There’s no universal percentage because some installations are effectively finished on day one while others acquire new equipment regularly. Even one or two spare U can be valuable in a small installation; more is sensible if you already expect the network to grow.

Buying a cabinet in which the final available U is occupied during the initial installation is wonderfully efficient — right until somebody buys one more switch.

In extremis, you can then find yourself replacing an entire wall-mounted cabinet because a device 44.45mm high has appeared.

A little unused rack space is usually much cheaper than replacing the enclosure later.