Is it okay/common practice to reuse reference letters? Prompted by this question about "exporting" a reference letter, I am now thinking about the question if it is okay or even common to re-use already used reference letters.

For a little context: I am in the process of applying for postdoc positions. I have requested and gotten very nice letters for one position and - as they were similarly relevant - have used those letters once more for another application, after asking the people who wrote them if they are fine with that.

I have, similar to the OP of the previously mentioned question considered what I should do when I get deeper into the process of applications and will write more and more of them. Would it be okay to just use the letters I already have over and over? In general, I think in my case those people would have nothing against that, but I still feel oblidged to at least let them know who they are "refering" me to, before they might get any requests for further information out of the blue.  Repeatedly pestering them every time I want to submit another application might get old fairly soon, though. 
I honestly have no idea if there is any common practice about this. As we all know, the academic job market is not that... easy, so it might be necessary to apply for quite a bunch of positions before getting one. I can't imagine that every new PhD keeps repeatedly asking the same people for letters until they find their position (asking different people once in a while would be an option, but then I would kind of "loose out" on the best letters by the people who know me best at least for some applications.), but I also would feel awkward "admitting" to have reused it to get a position without telling the writer about it beforehand.

To those in the position to write such letters:


Is it okay for you if they are reused without your direct knowledge?
Would you prefer to be asked every time or once ("Can I keep using this letter?")
Is there some option I completely miss out on?
How did you handle this previously (in case someone asked/reused them without asking and you were contacted/...)?
In the US, this practice essentially can't happen, because letters of reference are almost never given directly to the student—instead, the referee submits the letter directly to the requester. So you would have to ask your referees to submit the letter again on your behalf.

If you're in a country where the norm is to give a letter of reference directly to the applicant (such as Germany), the letter is usually given out without a target recipient, and is therefore much generic. In such cases, you do not need to request permission to reuse the letter, as it is already designed for such use. 

If you're in doubt, just ask the person who wrote you the letter. Most people will want to update a letter, particularly if it's several years old.