Tinning a soldering iron, how often to do during soldering?

I know you do it in the beginning and at the end of soldering. What about in the middle of doing some soldering? Do you need to clean it and re tin it again after you are done with one wire and are moving on? Can you use the tip tinned from the previous time to solder or do you have to clean it because it may be oxidized? Do you do all of this with the iron heated, or are there parts that the iron needs to be off? New to soldering, been doing research but still things I'm iffy about. Thanks!