Soldering Iron Tip Tinning
Leave the solder on the tip.
The tip of most soldering irons are iron plated copper.
As such, if the tip is exposed to the atmosphere, it will rust.
Now, typically, there is a layer of solder which coats the tip. However, if you wipe the tip off, the solder coating remaining is very thin, and can oxidize through, allowing damage to the actual iron plating of the tip. Leaving a heavy coating of solder on the tip (e.g. the "blob" the manual describes) is indeed a good way to preserve tip life.
However, the great majority of tip wear occurs when the iron is hot, as the increased temperature dramatically increases the rate at which the tip will oxidize and react with the solder, so it's not absolutely critical you leave solder on the iron tip when you turn it off. It's a good idea, but it's not absolutely vital.
It's also worth noting that the solder blob will probably protect the tip from light mechanical damage during storage as well.
During normal use, the mechanism that wears through the tip plating is different. Basically, over time, the iron plating actually dissolves away into the solder. However, there's not much you can do about this, short of using a fancy soldering system that reduces the iron temperature when it's in the stand, which reduces the rate at which the iron dissolves away (at least when it's in the stand).
Basically, iron tips don't last forever.
During normal use, you should never wipe the tip before placing the iron back in the stand. You should always wipe it after removing it from the stand, before actually soldering something.
- Remove iron from soldering iron stand.
- Wipe tip on sponge.
- Solder something
- Place iron back in stand with solder left on tip.
- Repeat.