I think generally it's Okay leaving it on Max. But there are a few things to consider.
1. If you leave it on max and fold 24/7 for a long time it might shorten the life span. Might.
2. If you plan of keeping the card beyond, say over two or three years. Lowering it a bit would be a good idea, just to be on the safe side.
3. The makers of the cards, and many but not all GPU OC applications are fairly conservative. After all, some cards are just simply sub-standard, just
scrape through final QT. Other "Golden cards," can go beyond the max, but not recommended for 24/7 continuous use (fah for example).
The point I am making in 3. is that no company (especially card manufactures, third party OC vs etc.) don't want lots of RMAs. In the case of OC
software, no RMA of course, but they don't want to be blamed for frying lots of GPUs.
4. In the end, after reading up about it (as you have) it's a personal choice, and depends on how conservative or willing to really push it one is, and
of course if the card overheats, it will need to be lowered. (core MHz, voltages, or both depending on the card.)