Re: Cleaning/oiling fretboard - how to with strings ON?
You can't really clean the fingerboard (well) unless you take all the strings off and while you can oil it with the strings on, it's a bit of a pain in the butt and you will (obviously) get oil all over your strings.
You won't hurt the neck in anyway by taking all the strings off for a short period (e.g. not years and year) of time. Will the neck flex a little when you take the strings off? Yes but it will also go re-flex when you put the strings back on.
In regards to tuning, it will be easier to get the guitar back in tune if you block the bridge (underneath the fine tuners) before you take all the strings off.
This is called as slandered answer. Thanks for your guidelines