It is common to have a slight rise on the strings, because they are more loose and vibrate more the further they are away from the nut. They are also easier to push down with the slide the further away from the nut, so need more room to keep from pressing down to the neck. That said, you don't want too much height rise or it may be difficult to sight-track the fret markings. But since most players I know use fret markings as just a general guide and play slide more by ear than sight... that may not be a big issue.
So my answer is: have the strings raise a bit, but not an incredible lot. Others may have additional insight to offer. I'm not a "slide" player (I can play slide, but I mainly use fingers)... so my experience with such is limited to well, my experience. ;D
I try to have an equal distance, like figure 1.
What Ted proves is back to what you said originally Rick: " i know wen it comes to cbg there are no rules"
That's the one rule I've found to be consistently true. Someone once tried to point out the 12th fret rule as set in granite (the distance from the nut to the 12th is the same as the 12th to the bridge)... and someone else then asked "Then why do we often slant our bridges, changing that distance?" In the end I've found it always comes down to "try it and see what you like best".
When examining off-the-shelf guitars we find that in every case the distance from the strings to the neck at the 20th fret is always greater than the distance at the 1st fret. So that would seem like a pretty set "rule"... except that we're making straight-board fretless necks. So when doing that what Ted says makes perfect sense: have the string high up enough off the neck at the first fret line for a slide to work, then just hold that distance all the way down.
You know what I'd check as an "authority" reference? Steel guitars. They're sliders. If you can find one nearby... see how their strings are set. That would probably be a pretty reliable indicator of what the industry has found to work.
But hey, I once built a guitar with the neck set at a diagonal on a 2-level cigar box (it had an "edge" all around) and the bridge installed half on the surface, half on the edge (had to cut an L in the bridge). Wound up having great tone... so I don't know as "flat vs slanted" is going to make a bit of difference. :D
I know! Make two identical gits, one flat, one slanted.... mwahahahahaaa...