My only comment is that any plant can only tolerate so much acidity so lime wouldn't be a bad idea for anything. If the mix dropped to an acidity of 5.0 then I would imagine it would need to be increased in some way.

I am not sure if lime is the best answer but I have occasionally used it with no adverse side effects that I could detect.

I use limestone and granite interchangeably in my potting. Limestone doesn't dissolve very much except in the presence of acid, so I consider it more of a buffer than a base.

Part of the equation is how much water is used in watering and the salt buildup of all kinds. If pots are flushed regularly, limestone is no problem at all.

It isn't so much that most orchids (and most paphs as well) are lime-haters it is that they prefer an acid condition. The lime is actually (should be) there to counteract an initially too acid situation. If the mix is presently neutral or only slightly acid the lime will pull it out of the acceptable zone right now and it will take a very long time before the slow creep towards a more acid condition makes the lime a useful item. Some organic peats and redwoods are very acid from go and adding lime in the mix is of benefit, not 'when' but 'right now'. So it really is a question of what exactly is the 'compost' made of?

H