I try to end up with an averagely-worn tyre as the spare (assuming you have a car with a full-size spare!) and make sure it's not too old as well.
Along the way, as others have said, I try to buy tyres in pairs; sometimes that means the spare gets binned along with the worst of the 2 tyres being replaced, the other becoming the spare if it's newer.

If I need to "even up" I have on occasion bought a part-worn tyre; these can be remarkably cheap, but you do need to be careful on condition.

I also never let tyres get anywhere near the legal minimum, usually replacing them at around 3 or 4mm.

And I try to keep all 4 the same brand and type (or very similar type), certainly on the same "axle".