There are 2 issues with running tyres at low pressures:
The first concerns tyre distortion at higher cornering speeds, the tread will move (according to grip available) more than usual leading to a lot of sidewall flexing, (and the ultimate is rolling the tyre off the rim , but I have never seen this happen even at racing speeds).
At "normal" road speeds and enthusiastic corning you might suffer from increased tyre shoulder wear.
The second is resistance to "compression type" punctures. If you drive into lots of deep potholes at speed the ability of the tyre to absorb the impact is reduced and it is possible (although very unlikely) that a particularly deep and sharp hole might just lead to the "Snake Bite" puncture that cyclists often report (if running tubes) where there are 2 almost identical punctures where the tube has been trapped between the rim and the road.
Personally my +8 runs at 24psi on Avon ZV5 205/60 x 15 all round, and I am happy with that. If you are happy at 18 psi then use that , but do be aware that the greater tyre distortion can lead to minor air escapes with tubeless tyres , so check the pressures MORE regularly.
Using 18 at the rear will make it more tail happy, especially in low grip situations, but as long as you are confident of catching it (and enjoying that) I wouldn't be overly concerned.
