If you want book performance, fly the book figures. Simple. But in initial training, particularly if runway length is not an issue, you see this "add five/ten knots for the folks back home" thing a lot. Because of the higher margins above the stall.
The reasoning I can think of (since the instructors couldn't really tell me) is that so we can use the same point in the windshield as the aiming point (which in my case we take 3" above the glareshield), since we are flying at the same speed, irregardless of flaps setting. Flaps is used to steepen the approach, but whatever you see at about 3" above the glareshield will still your aiming point. Actually, in this case I would call it impact point.
Not true. Deploying flaps has a number of effects:
- It increases the surface area of the wings, leading to greater lift (or equal lift at lower speeds)
- It changes the center of pressure, leading to an automatic pitch up or down
- It adds drag, leading to a reduction in speed
- It changes the average angle of incidence of the wing, initially leading to a pitch up, but eventually resulting in a lower fuselage angle of attack for the same wing angle of attack, and this results in a better view over the nose and a reduced risk of tailstrikes.
Which flap setting leads to which effect most prominently, depends to a large extent on the design but most of these factors will be present simultaneously somehow.
The last effect, the changed angle of incidence, will means that on flap deployment your "aiming point" will shift around the cockpit. They may have compensated for that by using an odd speed, but there is no guarantee whatsoever that this will work if the aircraft is loaded differently or if you start flying a different aircraft.
Working with an aiming point that's inside the cockpit in general doesn't work, since it changes with the speed, with the loading of the aircraft and is also dependent on head- and crosswind. To get a good descent you have to look at your "impact point" and the relative motion of the foreground/background vs. that point. If the sight picture around the impact point stays constant, and the background/foreground just gets bigger while you descend, you're going to end up at your "impact point". (And all you need to do further, is to prevent that impact.)