Why won't they need flattening? All stones wear out, usually in the form of a hollow.
Not if you spread the pressure.
But a bit of a hollow is usually no problem as long as you know it's there.
I had one mysterious stone with deep hollow lengthways along the coarse side which I guess must have been from regular axe sharpening - axe forcefully applied to stationary stone. No problem at all and seemed to help vaguely.
But the fine side had the opposite - slightly rounded over - so little that I didn't see it at first. I found it out when I noticed a plane blade developing a "reverse camber", very slight hollow in the middle of the blade. But then I just changed the way I used it - making more pressure towards the middle of the stone.
This was an 8" Norton "0" coarse and extra fine.
I guess the fine side was regularly used on the same axe, but hand held, which could round it over slightly. Stone applied to stationary axe.
I only ever flattened a stone once to see if it helped in any way. It didn't.
If you were caught out in the wild somewhere and had to sharpen something urgently you would soon find out how to do even on a spherical boulder!
I never bothered with water stones, I couldn't see the point, especially after following all the struggles the users seem to have with them - endless flattening, fast wear, water all over the place and rust! It seems to be just a passing fashion. Sooner it passes the better!