The Graphical User Interface (GUI), which is the primary interface between people and their computers these days, is most commonly an approximate implementation of the "desktop metphor". The basis of the metaphor is that the screen/moniter represents a clerical worker's physical desktop on which documents and folders of documents may be placed. There can also be desk accessories like a calculator. This metaphor is both useful and limiting. The metaphor has been extended to include things not normally on your desk, such as your office waste paper basket - the trash metaphor.
What is an office waste paper basket for? It is a receptacle into which you can toss documents (or folders of documents, or folders of folders of documents) for which you believe there is no need for retention. Without this receptacle you would eventually dissapear beneath a sea of useless documents. And let's just forget about things like the fact that you can also throw used polystyrene coffee cups in it, and that you can't throw your desk in it - they are true but are unneccessary complications for the pupose of this rant.
Of course if you had enough filing cabinets you could just file all your documents, but the more filing cabinets you have the more floor space you need - and floor space costs money. Also your filing system might start creaking at the seams - you could start spending more and more time retrieving the documents that you need to access - and time costs money. So you throw stuff in the trash, which works fine as long as you don't exceed its daily capacity. At the end of the day you leave your office and go home to your 2.4 kids and when you get back in the morning the trash has been magically emptied by those denizens of the night, the office cleaners. And if you make a mistake? Well if you realise your mistake before you go home then you just do a bit of rummaging (unlike if you had put it in an office shredder). If you don't realise until you wake up in the middle of the night in a cold sweat then things are more complex (you call security to ask them to ask the cleaners not to empty your trash, and if they already have then to keep the bag into which they emptied it). So basically, although there is some margin for error, you should be careful not to throw anything in there that you might want again, and if you realise that there is something in the trash that you might want again you should take it out straight away, in case you forget.
There are a few things about the Trash on a computer where there are significant differences between it and its physical counterpart. One is that it is huge. You could sit there throwing virtual documents in it for years and it would never fill up. For this reason you generally don't bother employing virtual office cleaners to empty it every night. It is also easily searchable (just like your virtual filing cabinets) so it is trivial to retrieve documents from it. This can lead to a blurring of the metaphor, with the Trash being thought of not as an office waste paper basket, but as a folder labelled "Trash".
So now where do we stand? It seems to me that you ought to make a decision. Do you use the Trash according to the metaphor or not? If so, then fine - and have a process that empties it every night. If not, then why use the Trash at all? Just delete documents (put them in the shredder) or put put them in folders. You could have a folder labelled "Trash" if you wanted, into which you put documents that you will probably want to shred at some point - but it is equivalent to a folder on your desk, not a waste paper basket on the floor.
Two bits of advice. Don't file important documents in your Trash and don't empty anybody else's Trash unless they are expecting it because they might be using it as if it were a folder. And get yourself a decent backup strategy too. Sorry, that's three bits of advice.
www.zenatode.org.uk Ian Gregory 2010