Yep, I know what you mean about higher giving more expression. I've also noticed that high makes string bending a little quiter and I'm less likely to catch another string when bending or doing vibrato. I would think you would use close to the same relief in all your guitars though wouldn't you?
Actually, each of my necks seem to have their own "sweet spots". Obviously there's always a tradeoff between playability and sound (tone, sustain...etc,). On every guitar there's a sweet spot where your action/relief and sound come together without too much sacrifice of either. For example, my RGT (Ultra Prestige) requires far less relief than the original wizard on my RG770. The 770 will start to choke fretted notes with the relief set like the RGT. The neck on my USA likes to be in between the RGT setting and the 770 even though, theoretically, it's the same neck as the 770.
Obviously there are differences between all of my guitars that will affect the "sweet spot" on each, but it does kind of show that you can't get one "perfect setting" that works for all of your guitars. Wood just isn't that exact. Two identical pieces cut from the same tree can, and usually do, have completely different properties. Multi-piece laminated necks are good at neutralizing those differences to some extent, but even they have different sweet spots.
There's a metric TON of players that think "the lower the better", and for some it might be true, but the vast majority of players could probably benefit from a slightly higher action, more relief, or both. Especially the ones that complain about their guitars being "thin" sounding. There's no good excuse for a thin sounding Ibanez.