Joking aside, I thought this might be worth a separate thread.
I think the main reason for retracting the flaps after parking is to make the aircraft less vulnerable to wind. Especially relevant if you park the aircraft for hours or days in a row. This applies for high-wing and low-wing aircraft obviously.
Another reason is that the wings are less exposed to someone or something bumping into them.
A third reason, maybe a little far-fetched, is that insects or birds can find a better spot to build a nest in when extended.
And, finally, It’s best to leave your aircraft as close as possible to a departure configuration which for most types is flaps up. Of course you will now all say “no need, that’s what checklists are for”. But be honest, have you never ever missed an item because you were distracted by something? I remember well a fully loaded C172 at Son Bonet taking off with full flaps and crashing. Pilot appeared to have been distracted.
The advantage, as already mentioned in other threads, is that it prevents someone from stepping on them in case of low-wing aircraft. This however depends on the aircraft type. Some have a wide enough entry area at the wing stub to make this a non-risk, and other types like the DA40 are entered and exited from the leading edge. Some types have split flaps.
I think this risk is mainly from passengers. It can be mitigated by the pilot putting the flaps down for entry and exiting and then retract them again before leaving her.
So, I’m going to be a good boy and follow the advice, err command, of the stern Navy pilot who taught me to fly. Unless you convince me otherwise!
Since I might be concerned by the famous flaps, I’ll share the very reason you might catch a glimpse of my bird with a nonchalant, aka flaps down position, whilst parked: it just makes boarding, and de-boarding, so much easier.
Being slightly vertically challenged, this in spite of my Dutch name, the ability to get as close as possible to the part of the wing I can step on, and taking hold of the roll bar to heave me up, is key to gain access to the appropriate seat.
Same as for those fancy sports cars (was eyeing and salivating in front of one of those sexy Caterham 7 a couple of days ago… sure could slide in, but getting out?), those of us flying sports airplanes kinda need some shoehorning to get in, and the use of a corkscrew to extract ourselves from our steed.
Seems getting better with age though 
And yes, you’ll find my bird’s feathers fully retracted during windy stays, alleviating any fly-away on it’s own tendency. Hopefully.
Ha ha. Talk about shoe horning yourself in; I used to fly with a friend in his Nicollier Menestral. It was great fun to fly but if getting in was a struggle, especially for a couple of old codgers one with dodgy knees, getting out was a comedy worthy of Mr Bean.
There were no flaps so no decision as to whether to leave them out or in.
On some aircraft, such as Zlin Z42 and Z43 series, flaps have to be fully retracted for taxiing but it is recommended to keep them fully extended when parked to prevent people from stepping on them.
On many airplanes – most types that I can think of – the intersection of the outboard flap, and inboard aileron is the perfect place to install aileron control locks, so a great reason to retract the flaps before securing the airplane. There are a very few types where hydraulic flaps are left extended, ’cause they are going to droop to there anyway, but that is type specific.
The solution is flaperons 
The PA28 allows pax to step on the flap (the pilot would never do that because it feels wrong to do so) but only if they are retracted. Of course if you want to discourage stepping on the flaps you would leave it down.
You don’t want pax attempting to step on the flaps of a Robin DR400, so the last action on shutdown is “flaps down full”. Then “flaps up” as part of the pre-start actions.
Regards, SD..
