I was taught to have full flap out by about 400 feet, but it wasn't until my first Chief pilot pointed it out that i realized that this practice is "unsafe" in an engine failure situation.
Question why it is unsafe. I assume most here are talking about a powered approach and any aircraft which loses power with or without flap is not going to make the powered aiming point, it will fall short.
Unless you are all conducting a glide approach from the start from some point late base and final you will not make the runway even clean unless you are aiming originally to land a good way in or at excessively high speed.
The benefit of having flap already out is that no configuration change is necessary and the touchdown speed off field will be much less.
Some larger and high performance singles have very little hope of making the runway even from a close downwind position, especially if there is a headwind to the field. The best method to teach is always know how far you can safely glide in any configuration and have a clear area to put down off airport in the event of total power loss. If you can safely make the runway do it, if not take the off field option. Focusing too much on stretching a glide back to the field results in stall spin accidents and its better to land in the rough upright (better still a reserve, padock or oval) at low speed than arrive at the boundary fence inverted and lacking control.
I think the CP should not even take off as at some point after lift off the aircraft would not be able to safely glide back to the runway after an engine failure.
Any sideslipping should be done with caution in most trainers, especially with lower fuel loads as it can lead to fuel interuptions.
In any case discussion on the point should also involve some practice. In a real emergency best to stick to what you know and have done rather than try something new.