It's good to see that manuals are suggesting the sling shot method. They're trying to teach good habits.
Using the slide lock as a slide release is fine for a one hand manipulation drill, but think about what's happening when you use it. Metal is being pressed against metal with a compressed spring. You then drag down the slide stop against the slide notch under this pressure to release the slide. Any time your grinding metal against metal there will be extra wear, especially with tension behind it. I don't think anyone lubes that area either, yet we lube every other sliding/rotating surface.
It can have a worse effect on some pistols than others. People do it enough where they eventually post threads like "My slide won't lock back on an empty mag." or "The slide releases by itself when I insert a new mag, even though it never did before."
The slide locks on some poly pistols like the XD, M&P, Glock and so forth, are just bent over sheet metal and are not as substantial as say a 92FS,1911 or BHP slide stop. This may be the reason some of those manuals will actually suggest the sling shot method but it's a good habit to get into because it allows that little bit of extra tension by pulling back the slide to chamber a round and it will be second nature to chamber a round weather a gun has an external or internal slide lock.
Lot's of things are done in movies that look neat like using the slide lock and flicking the cylinder over on a revolver, but they're not exactly great on the gun in real life.