It is a case by case basis...
My 642-1 says .38 +P on the barrel, which I used for years. When I converted to 9mm, I opted for standard pressure 147 grain ammo as my carry load (XTP). Being an Airweight, I wanted to keep the wear down on the frame. And I don’t need to hear about the pressure differences between .38 and 9mm... works fine for myself and quite a few other people.
Now, if a manual says don’t use it, probably shouldn’t use it (see below). If it says it increases wear, change recoil springs a little earlier than usual.
I remember watching a CZ SHOT Show booth review a few years back with a guy from CZ Custom. Was going over the differences between models, and his carry gun came up. When ammo was mentioned, he said he carries +P+. Was asked if it was rated for that... “all guns are rated for a few +P+.”
I feel that gives a better understanding of what higher pressure ammo does to a firearm. Most will not blow up with the first +P you fire, but if it doesn’t handle it... cracks will form and eventually it will fail (whether just stop working or blow up). If you increase wear, recoil springs start to wear out sooner... and the slide batters the frame quicker.
If the manufacturer says not to, I’m not saying to just blow them off and start shooting the hottest loads you can find. A lot of times, manufacturers put that in there for legal reasons. But do some research on forums, and see what people use in the model you have/one mechanically similar. If they put 1,000s of rounds of +P through them, go in cautiously. Let someone else be the guinea pig.