New to 3d printing. Tips on layer height and wall thickness

I am using cura right now on my new Lulzbot Mini (which I am LOVING).

I know what layer height does, but I am wondering why certain layer heights were selected for the lulzbot settings. The abs fine for example is set at 0.18mm layer height. That is an odd number. Does anyone know why it was selected?

As far as wall thickness, I see cura settings for wall thickness in mm. It makes sense I would want to make that number an even multiple of whatever a single strand thickness is. Should I make wall thickness a multiple of nozzle diameter? I suspect a single filament wall would be thicker than the nozzle diameter due to it expanding out as it is pressed into the previous layer (I know this is also impacted by head speed and extrusion speed/percent).

Thanks for any good rules of thumb, I suspect there is a GREAT wiki page on this somewhere that I have not seen yet.

Did you see the Mini Manual too?