...Peterson Strobe tuner...
I use the iOS Peterson strobe too...
Another vote for Peterson's iStroboSoft (on an iPhone, though they also have an Android version).
For the headstock clip-on crowd, Peterson's mighty StroboClip HD does *not* come with a "do not leave attached" warning. I checked with Peterson, and they said that, while they’ve never had a complaint about finish damage, that would be subject to the following:
Normal use of a clip-on tuner like the StroboClip will not have any effect, almost all of our "acoustic lab guitars" at Peterson have nitrocellulose finishes. However, avoid leaving any tuner on the headstock permanently, or for long periods (in the case etc.). This goes for any object such as capos, clip-on and suction-cup pick-ups and tuners.
Apart from any physical damage, nitro finish darkens with age when exposed to daylight. If there is an area which is covered or masked by an object attached to the guitar, that area will not darken in tandem with the remainder of the guitar's finished surface.
Due to the above and also due to the variations from luthier to luthier and brand to brand when it comes to finishes, we cannot be liable for any finish damage caused in whole or in part from prolonged attachment with the contact surfaces of the StroboClip.
In short, whatever it is, take it off your guitar after you're done playing. Don't "store" or "park" anything on your nitro-finished guitar.